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the capability of oscillations to probe the internal structure of stars is fully testified by solar studies .
for this reason , pulsating stars , that are spread in the hr diagram , are expected to largely improve our knowledge of stellar evolution and physical assumptions in models .
@xcite has shown that both radial and non - radial modes of pulsating stars are observationally distinguishable via the analysis of line profile variation ( lpv ) . from this pioneering work
, there has been an increasing interest in lpv modeling driven by the improvement in resolution and efficiency of modern spectrographs : @xcite included the influence of stellar rotation , @xcite considered photospheric spots , @xcite performed a mode identification via the moments of the line profile or @xcite via the doppler imaging technique .
@xcite noted that a pure kinematic approach is not enough for lpv modeling and that a realistic representation of the strength and broadening of spectral lines were necessary .
they considered the atmosphere of a pulsating star as a perturbed status of the hydrostatic equilibrium and represented the temperature perturbation of atmospheric layers with a series expansion in terms of effective temperature and gravity .
this operative method does not clearly represent the physics of @xmath0cep stars , pulsation modes do not change the energy production and the very small ( 1% ) stellar radius variation @xcite can not modify neither the effective temperature nor the surface gravity in an observable way .
however along a pulsation cycle , the compression and expansion of photospheric layers change the dependence of temperature and pressure on optical depth and let the effective temperature appear variable , with the pulsation period , if determined in a limited spectral interval .
a phenomenon indeed observed in some @xmath0 cephei stars by @xcite on the basis of a photometric approach .
these authors derived a t@xmath1 variability up to 1000 k with the pulsation period , from geneva colours .    with the density and temperature , the variation of atomic level populations
should also result in apparent surface gravity variations .
in fact , there is an equivalence between stellar surface gravity and electron pressure in the framework of spectral line formation @xcite .
a further contribution to the strength and profile of spectral lines comes from the microturbulence ( @xmath4 ) , that in principle could be modified along compression and expansion phases .    in this paper
we analyse a set of time resolved spectra of @xmath0 cep (=  hd205021  =  hr8238 ) , which is generally accepted to be the proto - type of a class of hot pulsating stars , with the aim of ascertaining the apparent variability of the effective temperature , gravity and microturbulence with the pulsation period .
these variations would strongly constrain any lpv method , as the one proposed by dupret and collaborators .
.for each spectrum we report the heliocentric julian date , the radial velocity measured from the si iii triplet at 4550 , the t@xmath1 and @xmath3 determined through the h@xmath0 line .
[ cols="^,>,^,^ " , ]     the number and equivalent widths of the silicon lines in our spectra are not enough to measure the microturbulence . however , the siiii triplet at 4552 , 4567 and 4574 is very sensitive to this velocity field and rules out a microturbulence range larger than 1.0 km s@xmath5 .
table[equiv ] shows that for the microturbulence variation deduced from nitrogen lines , the silicon triplet would present a 25% variability of equivalent widths much larger than the 7% observed ( fig.[var_ew ] ) .
we conclude that the microturbulence variability determined from nitrogen lines is a local minimum of the previous @xmath9 function and we assume a constant value for this velocity equal to the one we obtained from the average spectrum : @xmath4 = 8.1 km s@xmath5 .
we find that the effective temperature and gravity of @xmath0cep are variable with the pulsation period ( tab .
fig.[teff_var ] shows that t@xmath1 changes , almost sinusoidally from @xmath723700  k to 24400  k , in phase with the @xmath3 variability from 3.76 to 3.93 .    for a constant @xmath4 ,
the variability of equivalent widths with the pulsation period is also indicative of the temperature behaviour along a cycle .
fig.[var_ew ] shows that n ii and si iii lines are in phase and that they change out of phase compared to o ii and si iv .
we have computed the expected equivalent widths of the representative lines of fig.[var_ew ] for atmosphere models with 20000  @xmath10  30000 k , @xmath3 = 3.91 , @xmath4 = 8.1 km s@xmath5 and abundance listed in table 2 .
it appears ( fig.[teff_var ] ) that the observed behaviour of these lines is due to a variation of the effective temperature between 25000 and 26000 k. that is a result in agreement with @xcite who derived an effective temperature of 26000 k from the ratio of si iv and si iii line strength .
the top panel of fig .
[ teff_var ] shows the n ii abundance computed along the pulsational phase , considering the corresponding values of t@xmath1 and @xmath3 and keeping constant the value of @xmath4 ( 8.1 km s@xmath5 ) .
this reflects the variations of the ionisation state of nitrogen .
with the aim of determining the variations of the effective temperature , surface gravity and microturbulence velocity that occur in hot pulsating stars during the pulsational cycle , we performed time resolved r=20000 spectroscopic observations of the prototype @xmath0  cep along a complete period coverage .
as a first step , we confirm the t@xmath1 and @xmath3 found in a previous paper by @xcite and we measure @xmath4=8.1@xmath20.9  km  s@xmath5 , from a sample of single lines of oii . with these values , the abundances resulted to be solar for nitrogen , oxygen , neon , aluminium , silicon and sulfur , while slight under - abundances have been inferred for carbon , magnesium and iron .
we find that along a pulsation cycle , after a correction for the pulsational doppler shift , the h@xmath0 line profile changes are matched assuming a variability of the effective temperature in the range of 23700 to 24400 k and a variability of gravity in the range of 3.76 to 3.93 .
the amplitude of @xmath11  0.08  dex measured in the gravity variation could lead the reader to the erroneous belief that it corresponds to a radius variation larger than the one computed by @xcite , via cinematic considerations .
however , the variability of @xmath3 here determined for @xmath0  cep is not a consequence of changes in the surface gravity because of expansions and compressions of the star as a whole .
we ascribed the @xmath3 variability to a change of the electron pressure @xcite induced by temperature , that modulates the ionisation state of metals .
thus , it should not be linked directly to the radius of the star .
metal lines are characterised by equivalent widths variable with the pulsation cycle .
these are explained by a variable effective temperature between 25000 and 26000 k as a consequence of the changed ionisation state .
it could be that the most extended formation region of the h@xmath0 line is at the origin of the lower temperature , in agreement with the 15% larger amplitude presented by the radial velocity measured through the h@xmath0 core .        as to the possibility that the microturbulence depends on the compression and expansion of the atmospheric layers
, we find that it is constant within a 1.0 km  s@xmath5 .
we conclude that pulsations modify the dependence of the temperature and pressure on the optical depth in the atmosphere of @xmath0  cep .
this mimics a variability of the effective temperature and gravity when a small range of wavelengths is selected .
we hope that this information can help in modeling the variability of spectral lines that is not only due to the pulsation velocity field @xcite .    along the expansion phase
, the negative radial velocity is expected to increase up to a null value just in coincidence with the maximum of the stellar radius . at this phase of minimum compression
, the photosphere is expected at the temperature minimum .
differently , we observe a delay in the temperature minimum about one tenth of the pulsation period . according to @xcite ,
this extra - phase shift is due to the non - adiabatic condition that occurs in the pulsating layers .
the authors wish to thank luigia santagati for the english revision of the text .
kurucz r.l . , 1993 , a new opacity - sampling model atmosphere program for arbitrary abundances .
in : peculiar versus normal phenomena in a - type and related stars , iau colloquium 138 , m.m .
dworetsky , f. castelli , r. faraggiana ( eds . ) , a.s.p conferences series vol .
time resolved spectra of @xmath0cep yield the following average results : t@xmath1=24000@xmath2250  k , @xmath3=3.91@xmath20.05 and @xmath4=8.1@xmath20.9  km  s@xmath5 . 
 n , o , ne , al , si and s abundances are solar while c , mg and fe are slightly under - abundant . 
 pulsational amplitudes of @xmath6t@xmath1@xmath7700  k and @xmath6@xmath3@xmath70.2 dex are found from h@xmath0 . 
 the metal lines give similar amplitudes but centred on t@xmath1@xmath725000  k. an upper limit of 1.0 km s@xmath5 to the variability of the microturbulence is derived from the si iii triplet at 455  nm . 
 the radial velocity amplitude derived from the core of h@xmath0 is @xmath715@xmath8 greater than that from the metal lines .    
 latexl-.36em.3ex-.15em t-.1667em.7ex-.125emx    [ firstpage ]    stars : individual : @xmath0cephei  stars : early - type  stars : oscillations  stars : abundances