COMMISSION 27 OF THE I. A. U.
              INFORMATION BULLETIN ON VARIABLE STARS
                          Number 3542

                                                 Konkoly Observatory
                                                 Budapest
                                                 16 November 1990

                                                 HU ISSN 0374 - 0676

  HD20629: an X-ray selected optically variable Ap star (*)

   HD20629 (=SAO93386=BD+18d 459=GC 3971) is a suspected chemically
peculiar (CP) star (Renson et al., 1990) that has been
identified as the optical counterpart of the serendipitous
X-ray source EXO 031656+1853.4. It has been observed during a
program of systematic photometric and spectroscopic studies of
stars discovered in EXOSAT X-ray images (Giommi et al., 1988;
Cutispoto et al., 1990).
   Single dwarf stars in the range B7-A5 are probably not X-ray
emitters at levels greater than 10^27 erg/s (Rosner et al.,
1985) with the exception of some CP stars (Cash and Snow,
1982). Considering a value for the interstellar reddening
A_v between 0 and 2 magnitudes, we obtain a distance for HD20629
ranging from about 240 to 100 pc, respectively. Assuming an
optically thin line and continuum model for a solar abundance
plasma in collisional equilibrium (Mewe et al., 1985) the X-ray
luminosity of HD20629 is in the range 10^29.9 + 10^31.5 erg a s^-1
according to the assumed values for distance and absorption.
These values are very high, suggesting either that the CP star
is a real strong X-ray emitter or that the emission comes from
a chromosperically active late-type companion.
   Here we present the first results of spectroscopic and
photometric observations obtained at the European Southern
Observatory (La Silla, Chile).
   Moderate resolution spectra both in the Halpha and Ca II H and K
regions have been obtained at the 1.52m ESO telescope. High
resolution spectra of the Ca II H and Li I (6707 Angström) lines have
been obtained at the CES fed by the 1.4m CAT telescope. Several
metal lines are present in these spectra. For instance in the
region 4000-4300 Angström the Sr II 49078 Angström), Si I
(4128-31 Angström), Fe II + Ti II (4172-3, 4179 Angström), Si II (4198
Angström), Cr II (4207 Angström), Cr II + Fe II (4233 + 4253 Angström
lines are clearly detected, and the most appropriate spectral classification
of HD20629 seems to be A0SiSrCr. No evidence for duplicity has been found.

(*) based on data collected with the ESA X-ray Observatory
    EXOSAT and at the European Southern Observatory, La Silla,
    Chile.

                          [FIGURE 1]

 Figure 1) V light curve of HD20629. Phases are computed with
 the ephemeris HJD = 2448165.0 + 4.7 x E. The vertical bar shows
 the expected maximum error. Observations are in the instrumental
 system.


    Photometry has been obtained over the period 30 September-
11 October 1990 using the 1.0m ESO telescope feeding a
single-channel photon-counting photometer equipped with a
thermoelectrically cooled RCA 31034 GaAs photomultiplier tube.
HD20512 and HD20278 where chosen as comparison and check star,
respectively, and did not show variability. The instrumental
differential V light curve, in the sense HD20629 minus HD20512
corrected for atmospheric extinction, is shown in Figure 1.
HD20629 is clearly variable, with a period of 4.7 days. This
value, that has to be considered as preliminary, is well in
agreement with HD20629 being a CP of the SiSrCr subgroup
(Catalano et al., 1990). The amplitude of the V variability,
about 0.09 magnitudes, is also outstanding.
   Further photometric and spectroscopic observations are
planned in the near future in order to obtain a more complete
study of this interesting object.


           G. CUTISPOTO (1), G. TAGLIAFERRI (2), F.A. CATALANO (3)

          (1) Osservatorio Astrofisico di Catania, v.le A. Doria, 6
              I-95125 Catania, Italy
          (2) EXOSAT Observatory, SSD-ESTEC, Keplerlaan 1, 2200 AG,
              Noordwijk, The Netherlands
          (3) Istituto di Astronomia, Universitá di Catania, v.le A.
              Doria, 6 I-95125 Catania, Italy




REFERENCES:

Cash, W., Snow, T.P.: 1982, Astrophys. J. 263, L59 [BIBCODE 1982ApJ...263L..59C ]
Catalano, F.A., Renson, P., Leone, F.: 1990, in progress
Cutispoto, G., Tagliaferri, G., Giommi, P., Gouiffers, C.,
    Pallavicini, R., Pasquini, L., Rodono, M.: 1990, Astron.
    Astrophys. Suppl. Ser. in press [BIBCODE 1991A&AS...87..233C ]
Giommi, P., Tagliaferri, G., Angelini, L.: 1988, in "X-ray
    Astronomy with EXOSAT", R. Pallavicini and N.E. White (eds),
    Mem. Soc. Astron. Ital. 59, 33 [BIBCODE 1988MmSAI..59...33G ]
Mewe, R., Groneschild, E.H., van den Oort, G.H.: 1985, Astron.
    Astrophys. Suppl. Ser., 62, 197 [BIBCODE 1985A&AS...62..197M ]
Renson, P., Gerbaldi, M., Catalano, F.A.: 1990, Astron.
    Astrophys. Suppl. Ser., submitted [BIBCODE 1991A&AS...89..429R ]
Rosner, R., Golub, L., Vaiana, G.S.: 1985, Ann. Rev. Astron.
    Astrophys. 23, 413 [BIBCODE 1985ARA&A..23..413R ]