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supersoft x - ray sources ( sss ) were established as a new class of astronomical objects during the early years of this decade ( trmper  1991 , greiner  1991 , kahabka & van den heuvel 1997 ) and are thought to contain white dwarfs accreting mass at rates high enough to allow quasi - steady nuclear burning of the accreted matter ( van den heuvel  1992 ) .
the sources are highly luminous ( @xmath0 ergs s@xmath1 ) , but since their characteristic temperatures are on the order of tens of ev , much of the energy is radiated in the far ultraviolet or soft x - ray region of the spectrum , where the radiation is easily absorbed by the interstellar medium . because of this ,
only 2 close - binary galactic supersoft sources are known ( motch  1994 , beuermann  1995 ) , though there should be about 1000 in the milky way ( distefano & rappaport 1994 ) .
the situation is further complicated by the fact that supersoft x - ray binaries are highly time variable , both at x - ray and optical wavelengths ( greiner 1995 ) . the greatest number
are known in the magellanic clouds , but they are difficult to study because of their distance . the quest to find new ssbs has inspired several projects including comprehensive studies of deep rosat pointings .
so far , these were not generally successful , however .
recently , two other approaches to search for further members of the sss class have been attempted : ( 1 ) using a unique variability pattern : although most sss are variable in their x - ray and/or optical emission , the behaviour of several systems is distinctive .
data collected during the macho team s monitoring of the lmc has shown that the seemingly sporadic x - ray bright states of rx j0513.96951 , a well - known supersoft x - ray binary in the lmc ( schaeidt  1993 , pakull  1993 ) , are correlated with short - lived optical low states ( pakull  1993 , reinsch  1996 , southwell  1996 ) . as first suggested by brian warner
, the birthday of whom we are celebrating here , at the sss workshop in garching in 1996 , the 3 year macho light curve ( southwell  1996 ) suggests a strong similarity to the vy scl stars .
vy scl stars are a subclass of nova - like , cataclysmic variables which are bright most of the time , but occasionally drop in brightness by several magnitudes at irregular intervals ( bond 1980 , warner 1995 ) .
( 2 ) searching among unusual cataclysmic variables : one possible galactic source , v sagitae , has been suggested by studying the properties of several unusual cataclysmic variables ( patterson  1998 ) .
steiner & diaz ( 1998 ) note the similarity of @xmath2 other galactic systems to v sge .    in the following
, i will describe both strategies and review the evidence for transient , luminous supersoft x - ray emission in the two cataclysmic variables ( cvs ) v751 cyg and v sge during their optical low states .
full details of the correlated x - ray and optical observations of 7   have appeared elsewhere ( greiner  1999 ) , so we only summarize the relevant information here .
the distinct lightcurve of 0513  and its similarity to vy scl stars led us to decide to monitor the light curves of the known vy scl stars .
when 7  started to drop in brightness somewhere between 1 march and 11 march 1997 ( fig .
[ lc751 ] ) we performed a target - of - opportunity rosat hri observation ( 4660 sec ) on 3 june 1997 . a new x - ray source , rx j2052.2 + 4419 , was discovered within 1  of v751 cyg , at a mean count rate of 0.015 cts / s . during a second rosat hri observation on dec .
28 , 1997 the count rate and x - ray spectrum are nearly identical to the june values .
in contrast , 7  was not detected during the rosat all - sky survey on nov .
19/20 , 1990 giving a 3@xmath3 upper limit of 0.019 cts / s in the pspc .
in addition , it was also not detected during a serendipituous pointing on nov .
11 , 1992 providing an upper limit of 0.0058 cts / s in the pspc . on both occasions
7  was in its optical bright state .
this suggests an anti - correlation of optical and x - ray intensity in 7 .    a new method ( prestwich  1999 ) to extract reliable spectral information from hri data allowed to craft a response matrix for a given observation .
fits using this response matrix to all the source photons of v751 cyg show that simple black - body models with kt of a few tens of ev are consistent with the data , whereas higher temperature models ( 0.5 kev ) can be ruled out ( fig .
[ ufs ] ) . an iue observation performed in 1985 ( during an optical high state )
was used to derive the extinction towards 7  based on the broad absorption centered at 2200  : @xmath4 @xmath5 @xmath6 = 1.1@xmath710@xmath8 @xmath9 .
this implies a distance of the order of 500 pc . with this @xmath6 the x - ray spectral fitting gives @xmath10 ev ( see fig .
[ ufs ] ) . at this temperature ,
the bolometric luminosity on 3 june 1997 is 6.5@xmath710@xmath11 ( d/500 pc)@xmath12 erg / s .
thus , during its optical low state , 7  was emitting soft x - rays with a temperature and luminosity which confirm that it is a transient supersoft x - ray source .
the appearance of 4686   emission in optical spectra taken in sep .
1997 also indicates the presence of @xmath1354 ev photons .
cyg , like the other members of the vy scl star group , accretes at a few times 10@xmath14 yr@xmath1 .
if the mass of the white dwarf in 7  is small , this may allow nuclear burning , as the high x - ray luminosity suggests .
the 7  values of @xmath15 and @xmath16 are consistent , within the uncertainties of @xmath17 and @xmath18 with the relation @xmath19 found for 5 ssb ( van teeseling  1997 ) implying that , if nuclear burning is the correct interpretation of the x - ray flux during the optical low state , then nuclear burning may continue during the optical high state .
the discovery that 7  is a transient supersoft x - ray source arose from the similarity in the optical light curve of rx j0513.96951 and vy scl stars .
0513 ( schaeidt  1993 , pakull  1993 ) shows @xmath204 week optical low states which are accompanied by luminous supersoft x - ray emission ( reinsch  1996 , southwell  1996 ) .
it is generally assumed that the white dwarf accretes at a rate slightly higher than the burning rate , and thus is in an inflated state during the optical high state .
changes in the irradiation of the disk caused by the expanding / contracting envelope around the white dwarf have been proposed as explanation for the 1 mag intensity variation ( reinsch  1996 , southwell  1996 ) .
the explanation of the x - ray / optical variability of 7  could be similar to rx j0513.96951 : variations change both the photospheric radius and the disk spectrum .
if the white dwarf has a small mass , than photospheric radius expansion is reached at 1@xmath710@xmath21 /yr ( cassisi  1998 ) .
the explanation for the character of the optical and uv observations is not yet clear , but it seems certain that the illumination of the donor and disk play important roles in determining what we see . if the x - ray source during the optical low state indeed is very luminous one may expect a strong heating effect on the secondary as well as on the accretion disk .
the heating of the secondary in 7  is probably comparable to that in sss because the illumination depends on the ratio of companion radius and binary separation which is similar in both kind of systems .
unfortunately , no photometry has been obtained during the optical low state to immediately test for this effect in 7 though it is anyway not expected to produce a strong modulation due to the low orbital inclination .
the question of the illumination of the accretion disk has to be addressed separately for optical low and high state .
there is ample evidence in some vy scl stars that during the optical low state the accretion disk has vanished . though we have no direct evidence for this in 7  due to the lack of optical observations , the disk is certainly optically thin thus drastically reducing the efficancy of illumination . in the optical high state
the illumination depends on whether hydrogen burning stops or whether it continues on an inflated white dwarf at a temperature below the sensitivity range of rosat : if the burning stops then there are no soft x - rays which could be reprocessed .
if the nuclear burning continues , reprocessing may still not be strong because the amount of reprocessing depends on the size of the accretion disk ( orbital period ) and only for large disks also on the flaring of the disk .
for v751 cyg
the disk is so small that any flaring is probably insignificant compared to the angle which the white dwarf subtends with respect to the disk .
even if flaring were significant , reprocessing of the radiation from the white dwarf will begin to have a dominant effect on the local disk temperature ( king 1997 , knigge & livio 1998 ) if the white dwarf luminosity @xmath22 @xmath23 ( where @xmath24 is the albedo of the disk surface ) .
that is , a disk around a 1  white dwarf accreting at 10@xmath25 /yr will be dominated by reprocessing only if the white dwarf temperature is @xmath132@xmath710@xmath26 k @xmath5 17 ev .
this is seemingly just a value between the temperatures of sss ( 3050 ev ) and v751 cyg ( 15 ev ) .
thus , one difference of the systems could be that the disk in v751 cyg is not flared and therefore not dominated by reprocessing while the sss disks are flared and dominated by reprocessing and thus are optically much brighter than the vy scl disks .
it has recently been suggested ( steiner & diaz 1998 ; patterson  1998 ) that v sge ( and possibly also wx cen , v617 sge and ) have properties very similar to ssbs .
these suggestions are based on the following characteristics , shared by these four stars but rare or even absent among canonical cataclysmic variables : ( 1 ) the presence of both and emission lines , ( 2 ) a @xmath27/h@xmath24 emission line ratio @xmath132 , ( 3 ) rather high absolute magnitudes and very blue colours , and ( 4 ) orbital lightcurves which are characterized by a wide and deep eclipse .
v  sge has been the target of three dedicated pointed pspc and hri observations ( one of these splits into 3 separate observation intervals ) , and in addition is in the field of view of another pspc observation .
a detailed comparison of the optical states of v sge and archival rosat observations has shown that during optical bright states , v  sge is a faint hard x - ray source , while during optical faint states ( @xmath28 mag ) , v  sge is a ` supersoft ' x - ray source ( greiner & teeseling 1998 ) .
spectral fitting confirms that v  sge s x - ray properties during its soft x - ray state may be similar to those of supersoft x - ray binaries , although a much lower luminosity can not be excluded .
the model that has been suggested for 0513cannot explain the observational data of v  sge .
first , the optical brightness changes of v  sge are very rapid : both the faint-/bright - state transitions as well as the succession of different faint states may occur on timescales of 1day ( compared to the smooth decline of several days in 0513 ) .
such very rapid changes are only possible if the white dwarf envelope expands and contracts on the kelvin - helmholtz timescale and the mass of the expanding envelope is rather small ( @xmath29 ) .
such a small envelope mass is difficult to accept for a white dwarf with stable shell burning ( e.g. prialnik & kovetz 1995 ) .
second , the expected optical eclipse would become deeper when the system becomes brighter , opposite to what has been observed ( patterson  1998 ) .
it is possible to explain the different optical / x - ray states of v  sge by a variable amount of extended uneclipsed matter , which during the optical bright states contributes significantly to the optical flux and completely absorbes the soft x - ray component ( greiner & teeseling 1998 ) . a simple wind model for the recently observed radio flux density of v  sge implies a mass - loss rate of the order of 10@xmath30/yr ( lockley  1997 ) . with their ( assumed ) terminal velocity of 1500 km / s
this wind zone is completely opaque for x - rays up to 0.7kev , even if the wind is assumed to be circumbinary instead of arising from one component .
since the radio measurement has been obtained during optical high state , it supports the above described scenario .
as shown above , both strategies to search for new sss were successful .
this is very promising , and opens up the way to use optical observation strategies besides x - ray observations to identify new sss .
the surprising part of this development is that both , 7  and v sge , are short - period cvs , and if true , their transient supersoft source nature would establish a direct link between classical cvs and canonical sss which typically have orbital periods inthe 0.53 days range .
for both sources , v751 cyg and v sge , the estimates for their x - ray luminosity ( under reasonable assumptions on @xmath6 ) are @xmath20 10@xmath31 erg / s .
this is at the lower end of the stable burning region .
two issues are relevant in this respect : ( 1 ) as for most cvs , the available evidence for vy scl stars suggests that the white dwarfs have a low mass . at these low masses
the accretion rate necessary for steady - state burning ( consistent with fujimoto 1982 ) is 13@xmath710@xmath25 /yr ( sion & starrfield 1994 , cassisi 1998 ) , and correspondingly the luminosity is lower than the canonical values for sss .
( 2 ) even at rates below the steady - state burning level there could be a range where the hydrogen flashes are rather mild .
a recent study of this parameter space has shown that the luminosity between the flashes remains at the surprisingly high value of @xmath2010%30% of the burning luminosity ( rappaport 1999 ) .
_ acknowledgement : _
i m grateful to the organizers of this symposium for the exciting atmosphere which stimulated many fruitful discussions .
jg is supported by the german bundesministerium fr bildung , wissenschaft , forschung und technologie ( bmbf / dlr ) under contract no . 50or96098 .
i review the observational evidence for luminous , soft x - ray emission during optical low - states in the two cataclysmic variables v751 cyg and v sge , and discuss the possible link to the canonical supersoft x - ray sources .    
 = = = 1=1=0pt = 2=2=0pt [ 2]@series 7v751cyg 0513rxj0513.96951