COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3475

                                            Konkoly Observatory
                                            Budapest
                                            7 June 1990

                                            HU ISSN 0374 - 0676


        PHOTOMETRIC OBSERVATIONS OF THE WR STAR ROBERTS 93

   Presently more than 10 WR stars are known which are "WR + compact companion" 
system candidates (cf. Cherepashchuk and Aslanov, 1984 and references 
therein). The detection of strong enough (~10^36erg/s) X-ray radiation
due to accretion on the companion would be an important argument for 
binarity. However, for all the stars of this type observed with Einstein, X-
ray luminosity is less than ~10^33 erg/s (Moffat et al., 1981, Sanders et
al., 1981).

   Recently Hertz and Grindlay (1988) reported the detection of high
X-ray flux of the WC5 star Roberts 93. The X-ray to optical flux ratio is
fx/f0 (0.15-4.5keV)= 10^-1.7. The authors have suggested that this star is a
probable candidate to binary of considered type. However Pollock (1987) has
obtained the X-ray luminosity of this star L_x (0.2-4 keV)=1.4*10^33erg/s
(which is a typical value for single WR stars) from the same X-ray flux due
to the large interstellar extinction.
   Moffat and Shara (1986) carried out photometric observations of 
Roberts 93 in B filter (13 estimates on 13 nights in 1984) and made no direct
conclusion on the optical variability. The amplitude was < 0.04m, a period
of 4.6d fits the data best, but its significance is small.
   Our photometric observations of Roberts 93 were obtained on 29 nights
during a 42 day interval in 1989 June-August using the 60 cm telescope of
Sternberg Institute Maidanak Station. 151 individual light estimates were
obtained in V filter with a pulse counting photoelectric photometer. The
comparison stars c1, c2, c3 were used (Fig.1) with UBV magnitudes presented
in Table 1. The data are available on request from I.I.A. The light curve is
shown in Figure 2. The instrumental scatter was ~ 0.03m owing to the relatively 
small size of the telescope. A search for periodic variability in the
frequency range 0-2 day was made. The power spectrum was calculated as 
described in Doroshenko et al. (1985) and is shown in Figure 3. The maximum at
v1 = 0.017 day^-1 (P=59d) and its aliases (arrows) arise due to a small

            
           Table   I

Name   V       B       U        n
                                 
c1     12.69   13.89   -       11
c2     10.42   12.23   -       11
c3     10.21   10.64   10.82    3
1      13.39   14.16   14.39    1
2      13.36   14.46   -        1
3      13.49   14.84   15.72    1


                                  [FIGURE 1]

                                  [FIGURE 2]

   Figure 2. V magnitude difference versus time for Roberts 93

                                  [FIGURE 3]

                Figure 3. Periodogram of Roberts 93

                                  [FIGURE 4]

 Figure 4. B magnitude versus time for Roberts 93. Open circles are
 photoelectric magnitudes.


trend in the brightness of c1 (cf. curves Roberts 93 - c1 and c2 - c1 in
Fig.2). The same peaks are also visible in the c2 - c1 spectrum. c1, probably, 
is slightly variable. The maximum of the remaining peaks, at nu2 = 0.34
day 1, shows the confidence level of 4% if calculated according to Doroshenko 
et al. (1985). It means that in the case of a pure white noise process
the probability of occurrence of such powerful a peak is ~4%. Evidently,
this and other maxima are not significant with a usual confidence level of
1%. The amplitude of the corresponding harmonic component a (nu2)=0.016m.
Hence the full amplitude of regular variability is less than ~ 0.03m.
   The analysis of 25 plates from photographic archive of Sternberg 
Institute (the comparison stars 1, 2, 3 were used, cf. Fig.1, Table 1) has
shown that the light of the star was ~14.6m-14.8m (B) in 1949-1968 and 
became slightly brighter after 1973 (14.3m-14.5m (B)), (cf. Fig.4). Probably,
this points to the existence of long-term variability. One can note however, 
that photoelectric magnitudes of Hiltner et al. (1964) B=14.48m, Pyper
(1966) B=14.38m and our value (1989, B=14.41m) are in satisfactory agreement.

   Thus the lack of optical variability of Roberts 93 does not confirm the
Hertz and Grindlay's assumption.


I.I. ANTOKHIN, A.M. CHEREPASHCHUK, T.R. IRSMAMBETOVA, S.Yu. SHUGAROV

                    Sternberg State
                Astronomical Institute
                    Moscow, USSR


References:

Cherepashchuk, A.M., Aslanov, A.A.: 1984, Astrophysics and Space Science,
         102, 97. [BIBCODE 1984Ap&SS.102...97C ]
Doroshenko, V.T., Efimov, Yu.S., Terebizh, V.Yu., Shakhovskoy, N.M.: 1985,
         Izv. KRAO, 73, 143. [BIBCODE 1985IzKry..73..143D ]
Hertz, P., Grindlay, J.E.: 1988, Astronomical Journal, 96, 233. [BIBCODE 1988AJ.....96..233H ]
Hiltner, W.A., Schild, R.E., Jackson, S.: 1964, Astrophysical Journal,
         139, 763. [BIBCODE 1964ApJ...139..763H ]
Moffat, A.F.J., Firmani, C., McLean, I.S., Seggewiss, W.: 1981, in C.W.H.
         de Loore and A.J. Willis (eds.), "Wolf-Rayet Stars:
         Observations, Physics, Evolution", IAU Symp. No.99, p.577. [BIBCODE 1982IAUS...99..577M ]
Moffat, A.F.J., Shara, M.M.: 1986, Astronomical Journal, 92, 952. [BIBCODE 1986AJ.....92..952M ]
Pollock, A.M.T.: 1987, Astrophysical Journal, 320, 283. [BIBCODE 1987ApJ...320..283P ]
Pyper, D.M.: 1966, Astrophysical Journal, 144, 13. [BIBCODE 1966ApJ...144...13P ]
Sanders, W.T., Cassinelli, J.P., Van der Hucht, K.A.: 1981, in C.W.H. de
         Loore and A.J. Willis (eds.), "Wolf-Rayet Stars: Observations,
         Physics, Evolution", IAU Symp. No.99, p.589. [BIBCODE 1982IAUS...99..589S ]