COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1334

                                  Konkoly Observatory
                                  Budapest
                                  1977 September 7




           RS Cha : A DELTA SCUTI VARIABLE



    Because of its spectral type, RS Cha (=HR 3524=HD 75747)
was observed as part of a programme being carried out at Mount
John University Observatory to detect new delta Scuti type variables. 
It has been found to show light variations of variable
amplitude.
    The star was observed on 9th March, 1977 in the B and V
bands of the UBV system with the 61 cm telescope of the University 
of Pennsylvania at Mount John University Observatory. A
thermoelectrically cooled EMI 6094B photomultiplier tube was
used for the observations, each observation consisting of four
10 second integrations. Extinction was determined from the 
comparison stars and the observations were reduced to the UBV 
system. The comparison stars used were HD 75416 and HD 75505.
    Observations obtained in the B filter on the 9th March are
shown in the diagram. The magnitude differences for the 
comparison stars are also shown for comparison, their mean scatter
being 0.004 magnitudes.
    RS Chamaeleontis was discovered to be an eclipsing variable
by Strohmeier (1964). Andersen (1975) has determined the various
elements and physical parameters for this system and believes
the secondary to be an Am star.
    The lights variations exhibited by RS Cha show that it is
also an intrinsic variable, but the limited number of observations 
obtained on the 9th March have prevented an accurate determination 
of the period. From variations in the B and V filters 
we can only conclude that the dominant period lies between
0.074 and 0.097 days. Present observations are insufficient to
resolve which component is pulsating or whether both are.

                       [FIGURE 1]

  Magnitude differences for RS Cha (HD 75747) and comparison
  stars in B filter.

    If the secondary is in fact a classical Am star then it
is unlikely to show pulsational instability, (Breger, 1970). If
it is assumed that only the primary is pulsating and it is a
delta Scuti star, then by using the physical parameters of Andersen
and the P-L-C relationship for delta Scuti stars, (Breger and Bregman, 
1975) we find P0= 0.070 days. On the other hand, using the
P-L relationship of Dworak and Zieba (1975) for faint delta Scuti
stars we obtain P0 = 0.078 days. Because of the uncertainties
involved in the above relationships, these results are in 
reasonable agreement with the observed period range.
    For the period range in question the observed pulsation
constant Q agrees with the theoretical value of Q0 = 0.0335 days
given for the delta Scuti models above the main-sequence, (Jorgensen
and Petersen, 1974). Applying the period-mean density relationship 
with Q0 = 0.0335 days to the primary, a fundamental period
of 0.077 +- 0.006 days is obtained. This is in good agreement with
the results of the empirical P-L and P-L-C relationships.

    The observed light variations of small amplitude and period,
together with the good agreement of the physical parameters of
the components with mean delta Scuti properties, (Baglin et al.,
1973), do indeed suggest that at least one component in this system 
is a delta Scuti star.

    Further observations are planned in order to obtain a
more accurate period, to resolve which component is pulsating
or whether both are, and to determine whether non-radial pulsations 
are evident, (Fitch, 1975).





                           C.J. McINALLY,
                           R.D. AUSTIN

                           Mount John University Observatory, 
                           Lake Tekapo,
                           New Zealand




References:
Andersen, J., 1975, Astron.Astrophys., 44, 445 [BIBCODE 1975A&A....44..445A ]
Baglin, et al., 1973, Astron. Astrophys., 23, 221 [BIBCODE 1973A&A....23..221B ]
Breger, M., 1970, Ap.J., 162., 597 [BIBCODE 1970ApJ...162..597B ]
Breger, M., and Bregman, J.N., 1975, Ap.J., 200, 343 [BIBCODE 1975ApJ...200..343B ]
Dworak, T.Z., and Zieba, S., 1975, IBVS, No. 1005
Fitch, W.S., 1975, in Multiple Periodic Variable Stars
         IAU Colloquium No. 29 (Budapest), 167 [BIBCODE 1976ASSL...60..167F ]
Jorgensen, H.E., and Petersen, J.O., Astron.Astrophys. 35, 215 [BIBCODE 1974A&A....35..215J ]
Strohmeier, W., 1964, IBVS, No. 55