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in disk - dominated cataclysmic variables ( cvs ) , mass accretion onto a white dwarf ( wd ) from a relatively normal secondary star is mediated by a disk that extends close to the surface of the wd .
all cvs vary , but
the character of the variability probably reflects the time - averaged accretion rate from the secondary ( see , e. g. * ? ? ?
systems with low accretion rate show semi - periodic outburst of 3 - 5 magnitudes ( in @xmath0 ) and are known as dwarf novae ( dne ) .
these outbursts are due to a thermal instability that converts the disk from a low temperature , mostly unionized , optically thin ( in the continuum ) state to a high temperature , ionized , optically thick state . during the outburst , the mass transfer rate @xmath1 in the inner disk rises from @xmath2 to , and , in terms of the simplest picture of the system , the dominant source of fuv emission changes from being the wd to the disk .
by contrast , systems with high mass - transfer rates remain in the high state most of the time , and are known as nova - like variables .
finally there are `` z cam '' objects , which undergo normal dn outbursts as well as outbursts that stall during the transition to quiescence for weeks to months at an intermediate magnitude ( typically 0.5 - 1 magnitudes below peak ) .
these are thought to be systems with intermediate rates of mass transfer from the secondary star .    in this review ,
i would like to highlight recent progress toward understanding these systems as a result of observations with , , and the hopkins ultraviolet telescope ( hut ) .
as first shown using , the fuv spectra of disk - dominated cvs in outburst show `` blue '' continua that can be reasonably approximated in terms of a weighted set of stellar atmospheres , where the weighting is determined by the temperature and gravity of a steady state accretion disk and where each spectrum is broadened to mimic that of a rotating disk annulus . a modern example using data from  and is shown in fig.[v3885sgr_disk ] .
when , as in this case , the distance and inclination are known , the fit depends almost entirely on the observed flux . in these cases , the fact that the model qualitatively reproduces many of the features in the spectrum suggests that the derived @xmath1 is close to the correct value . on the other hand , models based on stellar atmospheres
have generally been shown to fail to reproduce the spectrum over large wavelength ranges , particular if the range spans the balmer limit .
stellar atmospheres have a pronounced balmer jump in contrast to what is observed in high state disk systems .
this is surely due to differences between disk and stellar atmospheres .
ad hoc solutions , such as including a transition region above the surface of the disk , are capable of addressing the problem but do not discriminate between likely mechanisms , which include viscous energy dissipation in or illumination of the disk atmosphere . to date there have been very few published attempts to create more physically correct model disk spectra and perform detailed comparisons to high quality spectra .
however , this is likely to happen soon .
the machinery does exist @xcite and model grids are beginning to be created ( see , e. g. * ? ? ? * ) .
the disk model fits will not provide a complete solution , however , because fuv spectra of high state spectra show clear evidence of winds .
p cygni like profiles in n v , si iv and ( most commonly ) c iv are observed in some systems , and the centroids of these lines are blue - shifted in others .
blue edge velocities of 2000 - 5000 @xmath3 are observed .
recently , a number of systems have been observed in the 900 - 1185   range with .
these show surprising diversity . in particular , although some systems do show high - velocity wind emission from s vi and o vi , it is common to see relatively narrow lines with small ( 100 @xmath3 ) blue - shifts from intermediate ionization states @xcite .
the velocity widths of these lines are too narrow for either the disk or the high - velocity outflow represented by c iv in the  range .
shortly after winds were first discovered in cvs , observations of eclipsing systems showed changes in profiles shapes that indicate substantial rotation , suggesting a disk origin for the outflow @xcite .
consequently , our basic picture of the high - velocity winds is of a bi - conical flow emanating from the inner disk .
@xcite were the first to attempt to model the profile shapes of wind lines as observed in high state cvs in terms of a kinematic prescription for a bi - conical wind .
they found that the -derived ( r=200 ) c  iv profiles of three systems  rw sex , rw tri , and v sge 
could be reproduced with moderately collimated winds with mass - loss rates ( @xmath4 ) of order 10% of the disk accretion rate ( @xmath1 ) and terminal velocities of 1 - 3 times the escape velocity at the footpoint of each streamline .
subsequently , @xcite succeeded , using a different parametrization for a bi - conical flow , in reproducing the c  iv profile of ux uma though an eclipse as observed at r=2000 with /ghrs .
this analysis was the first attempt to model changes in the profile through eclipse , and suggested , at least in ux uma , the existence of a relatively dense , high - column - density , slowly outflowing transition region between the disk photosphere and the fast moving wind .
recently , my collaborators and i have developed a new monte carlo radiative transfer code that invokes a sobolev approximation with escape probabilities to follow photons through an axially symmetric wind .
`` python '' is designed to produce a complete spectrum of disk dominated cvs @xcite .
emission sources include the disk , the wd , a boundary layer and the wind itself .
the flow geometry is defined either in terms of the parametrization of @xcite or @xcite .
the code consists of two separate monte carlo radiative transfer calculations . in the first
, the ionization structure of the wind is calculated using a modified on - the - spot approximation . in the second ,
a detailed spectrum is calculate for a specific wavelength range of interest .
a fair summary of progress to date is as follows : mc methods , such as python , can be used to obtain spectral verisimilitude to individual lines fairly easily .
an example of this is shown in fig .
[ ixvel_hut ] .
systematic searches are now needed to determine whether one can actually model all the lines with a single wind geometry , and to determine how well one can do on average .
several of us are beginning this effort .
if this effort is successful , we will be able to determine the basic parameters , such as the mass - loss rate , degree of collimation , and ionization state , of the winds of high state cvs . until this is done , it is difficult to address many of the physical questions that need to be answered , including whether the wind is radiatively - driven , as is usually assumed .
the observational and theoretical evidence for this is murky .
if the wind is radiatively driven , one would expect that the wind lines would be strongest when systems are brightest . but @xcite found no correlation between the strength of wind features and continuum brightness in /stis observations of two nova - like variables , ix vel and v3885 sgr . and @xcite and @xcite have argued that the luminosities of cv disks are at best marginally sufficient to accelerate a high - velocity wind .
hydrodynamical simulations of radiatively - driven cv winds have recently been carried out , but comparisons of the theoretical predictions to observations have produced mixed results ( see e.g. * ? ? ?
thus , alternative wind - driving mechanisms must be ( re)considered instead of , or in addition to , radiation pressure .
these include viscous heating of the upper disk atmosphere @xcite , and irradiation of the disk @xcite as well as magneto - centrifugal forces leading to constant angular velocity rotation out to the alfvn surface @xcite .
knowledge of the kinematic structure of the wind would be a strong discriminant between them .
the appearance of quiescent systems is quite different from those in outburst , and fairly varied . at one extreme are systems , such as u gem , with  spectra ( fig .
[ ugem_sscyg ] ) characterized by broad lyman absorption lines and narrower metal absorption features .
these systems can be modelled , sometimes in great detail , with synthetic spectra of metal enriched wd - photospheres .
the metal enrichment is understood to be a result of on - going accretion . at the other extreme
are systems dominated by broad , fairly symmetric emission features .
spectra of ss cyg , also shown in fig .
[ ugem_sscyg ] , provide a good examples of this type a system .
the dominant continuum emission in these systems is not well - understood , although the lines almost surely arise from the disk ( see , e. g. * ? ? ?
since in quiescence the bulk of the disk is cold and unionized , explanations typically involve surface layers of the disk or a chromosphere , heated via x - ray illumination or magnetic flares in the disk .
the emission region is unlikely to be optically thick in the continuum since that would limit the physical size of the emitting region to that of the wd .
most systems , e.g. vw hyi @xcite , lie between the two extremes , with contributions from the wd and from the `` second component '' .
the basic questions one would like to answer about quiescent systems depends on the nature of the emission . for the wd systems , one would first of all like to establish the fundamental parameters  temperature , radius , abundances , rotation rate , gravitational redshift  of the wd .
in addition , one would like to explore the effects of accretion onto the wd .
for the other systems , one would like to understand the real nature of the `` second component '' and what it implies about the structure of the disk ( and the nature of the wd ) .     of about 40,000k to 30,000k .
[ ugem_sscyg],title="fig:",width=240 ] of about 40,000k to 30,000k .
[ ugem_sscyg],title="fig:",width=240,height=240 ]    as an example , i would like to describe the analysis of multiple fuv spectra that were obtained of wz sge following its outburst in july 2001 @xcite .
wz sge , at 42 pc , is the closest known cv , and possibly the closest accretion disk system . among cvs ,
wz sge has one of the shortest binary periods ( 88 m ) , and one of the longest interoutburst periods ( 20 - 30 yrs ) .
and  observations during the outburst showed a high excitation spectrum dominated by broad o vi , nv , and c iv features , consistent with the high ( 75@xmath5 ) system inclination @xcite . following the outburst ,
the spectrum acquired the characteristics of a wd - dominated system ( fig .
[ wzsge ] ) . from 2001
september to 2003 march , the spectra show a steady decline in the flux , a steady decrease in the color temperature , and a gradual increase in the width of , all of which are consistent with a long term cooling of a wd .
a detailed analysis shows that @xmath6 declined from 24,300 in 2002 october to 15,900 k in march 2003 .
the flux level in 2003 was still well above that of the pre - outburst system , indicating that the wd was still cooling . assuming a normal wd mass - radius relationship and a distance of 42 pc
, it was possible to show that all of the post - outburst spectra are consistent with a wd mass of 0.9 , as had been inferred previously by more indirect means .
this result is interesting in part , because in some other systems ( e.g u gem ) , simple 1-t models do not produce constant radius with time , which has lead to suggestions of non - uniform surface temperatures of the wd or a declining `` second component '' .      following the 2001 outburst @xcite . [ wzsge],title="fig : " ]    one of the puzzles in the case of wz sge
is why the post - outburst energy release has been so large ; the post - outburst excess energy release is roughly 15% of the outburst energy .
there are two basic possibilities .
first , there could be on - going accretion .
this is consistent with the fact that we continue to see metal lines in wz sge .
the time scale for metals to settle in a wd are of order a few days .
therefore , if the metal lines in wz sge arise from the photosphere , then there must be ongoing accretion .
secondly , the wz sge outburst both heated and left the wd with extra mass on its surface , leaving the wd out of equilibrium .
the interior of the star has to respond to these facts , through a process known as compression heating .
@xcite argued that the cooling that is observed in wz sge could be modelled either as a 1.2 wd which accreted at during the outburst with no accretion afterward , or with a 1 wd accreting at during the outburst and following the outburst .
the latter possibility is most consistent with other known facts about wz sge .
the spectra obtained with , , and hut represent qualitative improvements over those that could be obtained with . for the high state spectra ,
the surprise in the spectra is the plethora of lower ionization state narrower lines , especially in higher inclination systems .
these lines most likely arise from a transition region between the disk and the high velocity wind , another indication that this zone can not be ignored . with it was clear that wds emission dominated the emission from some low state spectra . with  and , it is possible to model not only the temperature evolution of wds , but to determine abundances in the photosphere and to begin to characterize the second component in the emission .
the existing spectra demonstrate the importance of multiple observations of individual systems . sadly , unless a servicing mission provides  with a revived uv spectroscopic capability or the operations staff
are able to restore  to operational status , the prospects for obtaining new spectra are modest .
acknowledgement : this work was supported by nasa through grants associated with analyses of  and  spectra of cvs , specifically go-9357 and go-9791 from the space telescope science institute , and nag5 - 13706 and nag5 - 13717 directly from nasa .
and  have provided high signal - to - noise , high - resolution spectra of a variety of cataclysmic variables and have allowed a detailed characterization of fuv emission sources in both high and low states . here , 
 i describe how this has advanced our understanding of non - magnetic cvs and the substantial interpretive challenges still posed by the observations . in the high state , 
 the fuv spectra are dominated by disk emission that is modified by scattering in high and low velocity material located above the disk photosphere . 
 progress is being made toward reproducing the high - state spectra using kinematic prescriptions of the velocity field and new ionization and radiative transfer codes . in conjunction with hydrodynamical simulations of the outflows , accurate estimates of the mass loss rates and determination of the launching mechanism 
 are likely forthcoming . in quiescence , 
 the fuv spectra reveal contributions from the wd and the disk . 
 quantitative analyses have lead to solid measurements of the temperatures and abundances of a number of wds in cvs , and of a determination of the response of the wd to an outburst . 
 basic challenges exist in terms of understanding the other components of the emission in quiescence , however , and these are needed to better understand the structure of the disk and the physical mechanisms resulting in ongoing accretion in quiescence .    
 accretion disks , cataclysmic variables , dwarf novae , white dwarf