COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                      Number 2721

                                          Konkoly Observatory
                                          Budapest
                                          15 May 1985

                                          HU ISSN 0374 - 0676

       PHOTOMETRIC OBSERVATIONS OF THE RS CVn BINARY sigma CrB

  The bright component (HD 146361) of the visual binary sigma CrB (ADS 9979 AB) is
a double - line spectroscopic binary with period of 1.14 day (Batten et al. 1978)
which has all the characteristics of an RS CVn system. It shows intense
chromospheric, transition region and coronal activity as seen in the optical,
UV and X-ray bands (Young and Koniges 1977, Tarafdar and Agrawal 1984,
Agrawal et al. 1980). Bopp (1984) has recently concluded that sigma CrB is the
most chromospherically active main-sequence system once observed.

  Based on their photometric observations Skillman and Hall (1978) concluded
the presence of a wave-like distortion with an amplitude of 0.05mag (V) with a
minimum at 0.4 phase. They also found evidence for a short-time variability
similar to that of delta Scuti type stars superposed on the distortion wave. In
order to verify this result we carried out photometric observations of this
system with the 1.2 m telescope of Japal-Rangapur Observatory in the UBV
bands during 1981 and 1982 using 20 arc second diaphragm.

  We observed this system on 11 nights in UBV passbands and used HR 6043 and
HR 6108 as comparison and check stars, respectively. The photometric equipment,
method of observations and the reduction techniques employed for deriving
Delta m (variable minus comparison) have been described in a previous paper
(Vivekananda Rao and Sarma, 1983). The r.m.s. error of Delta m (check minus
comparison) was found to be approx. 0.02m in UBV passbands indicating that the
comparison star is constant in brightness during the period of observations
within these limits. The phases of observations are calculated using the
following ephemeris given by Tanner (1949).

             JD (Hel) = 2423869.105 + 1.139789dE

Figure 1 shows the plot of Delta m versus phase for the observations obtained in
the UBV passbands. Unfortunately our observations do not cover the phase
range 0.45 to 0.75. Due to this gap in the light curves, no firm conclusion
regarding the presence of a wave-like distortion in this system can be made.
However, from the remaining phase coverage (0.00 to 0.20, 0.30, 0.45 and
0.75 to 0.00) we conclude that within the limits of observational errors sigma
Cr B is nearly constant in brightness in each of the three passbands.


                            [FIGURE 1]

 Figure 1: Observed values of Delta m in UBV bands plotted as a function of
           binary phase using the ephemeris of Tanner (1949)

                            [FIGURE 2]

 Figure 2: Photometric data in UBV bands for three of the nights.





  To determine the nature and periodicity of the delta Scuti type variability
if any, we have monitored this system on a few nights for more than 2.5 hrs.
continuously. These observations are shown in Figure 2(a)- 2(c) for 3 of
the nights. From these figures it is very clear that sigma Cr B does not exhibit
any delta Scuti type variability of period 0.1 day except for a small scatter
which is comparable to the spread in the (check minus comparison) observations.
Skillman and Hall (1978) suggested the possibility of the comparison star being
the source of variations in sigma Cr B. They could not confirm this in the absence
of a check star. However, during the present observations, we used the same
star for comparison (HR 6043) as was used by Skillman and Hall (1978 ) along
with a suitable check star (HR 6108). From our (check minus comparison) star
observations it was found that the comparison star used by us and Skillman
and Hall (1978) was constant in brightness within the limits of observational
errors (+\- 0.02mag). Hence, the suspicion of Skillman and Hall (1978) about the
possible variable nature of the comparison star is not confirmed by our
observations. We conclude by stating that there is no indication of any delta Scuti
type variations in our observations of sigma Cr B.
  We, however, suggest that further photoelectric observations of sigma CrB with
small diaphragms should be made to study the variable nature of HD 146361.
Such observations obtained over long periods will be valuable in interpreting
the distortion wave properties of HD 146 361 and its similarity to other
members of the RS CVn group.
  PVR and MBKS wish to acknowledge Indian Space Research Organization (ISRO)
for financial support during this work. PCA and MVKA thank F. Velani for
technical support during the observations.


 P. VIVEKANANDA RAO, M.B.K. SARMA,     P.C. AGRAWAL, M.V.K. APPARAO
 Centre of Advanced Study in           Tata Institute of Fundamental Research
 Astronomy, Osmania University,        Colaba,
 Hyderabad 500 007. India              Bombay 400 005. India


References:

Agrawal, P.C., Riegler, G.R., and Garmire, G.P., 1980, M.N.R.A.S., 190, 853. [BIBCODE 1980MNRAS.190..853A ]
Batten, A.H., Fletcher, J.M., Mann, P.J., 1978, Publ. Dominion Astrophysical
        Obs. Vol. XV, No. 5. [BIBCODE 1978PDAO...15..121B ]
Bopp, B.W., 1984, Ap.J.Suppl., 54 387. [BIBCODE 1984ApJS...54..387B ]
Skillman, D.R., and Hall, D.S., 1978, Inf.Bull.Var.Stars, No. 1529.
Tanner, R.W., 1949, Publ.Dominion Astrophysical Obs. 1, 473. [BIBCODE 1949PDDO....1..473T ]
Tarafdar, S.P., and Agrawal, P.C., 1984. M.N.R.A.S., 207, 809. [BIBCODE 1984MNRAS.207..809T ]
Vivekananda Rao, P., and Sarma, M.B.K., 1983, J.Astrophys. Astr., 4, 161. [BIBCODE 1983JApA....4..161V ]
Young, A., and Koniges, A., 1977, Ap.J., 211, 836. [BIBCODE 1977ApJ...211..836Y ]