COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2047

                                Konkoly Observatory
                                Budapest
                                1981 December 1

                                HU ISSN 0374-0676




      NEW OBSERVATIONS OF THE DELTA SCUTI VARIABLES SIGMA

               OCTANTIS AND B OCTANTIS.



SIGMA OCTANTIS

   The A7n star sigma Octantis (= HR 7228 = HD 177482) was first recognized as
a delta Scuti variable by McInally and Austin (1978). Our observations of 
this star were made on the 17th and 19th July, 1981 using a photoelectric
photometer attached to the 41 cm telescope of the Monash observatory. The
standard star chosen, o Octantis (= DM -89 1 = SAO 258218), proved to be
constant over the two nights observations, while sigma Octantis varied with a
single period of 0.097 day and a visual light range, DeltaV = 0.025 magnitudes
(c.f. results of McInally and Austin: period = 0.100 day, DeltaV = 0.03 
magnitudes).

Figure 1 shows the data collected and the reconstructed light curve from
Fourier analysis of the data. Our results indicate that sigma Octantis is
probably a delta Scuti star undergoing radial pulsation in the fundamental 
mode only.

                            [FIGURE 1]


B OCTANTIS

  B Octantis (= HR 8294 = HD 206553) was first reported as a delta Scuti
variable by McInally and Austin (1978). On the basis of two nights
observations they reported a period of 0.064 day, a visual light range of
0.010 magnitudes, and noted that the star exhibited a highly variable
amplitude.



  Using the 41 cm telescope at the Monash Observatory, B Octantis was
observed on the 17th and 19th July, 1981 (see Figure 2). Fourier analysis of
the data obtained revealed a period of 0.063 day and a visual light range of
0.010 magnitudes; this is in excellent agreement with the results of Mclnally
and Austin. In addition another period of 0.143 day with a visual light range
of 0.014 magnitudes was located. The standard star chosen was o Octantis
(= DM -89 1 = SAO 258218); both standard and variable lie within 1.5d of the
south celestial pole, hence the air mass through which they are observed
remains virtually constant. Sigma Octantis gave a constant signal throughout both
nights.

                            [FIGURE 2]


  B Octantis was again observed on the 9th, 14th and 20th September, using
the same equipment. However we altered the observing technique by measuring
B Octantis continuously, interspersed at about half-hour intervals with
single (2 minutes) observations of the standard. Fourier analysis of these
data again revealed the same periods. Figure 3 gives the data obtained and the
reconstructed light curves from Fourier analysis of these data.

                            [FIGURE 3]


   These two periods give a period ratio of 0.44 which cannot be attributed
to any commonly observed modes of radial pulsation. Breger (1979) gives the
observed period ratios for radial pulsation in delta Scuti stars as P_1/P_0=0.76,
P_2/P_0 = 0.60, P_2/P_1 = 0.81, and P_3/P_2 = 0.845. Breger (1979) also points out
that non-radial modes of pulsation have only been well established for a few
delta Scuti stars. Because the light variations of B Octantis are small, further
observations may not reveal more structure in the light curve. We plan to
measure colour indices of this star in an effort to locate its exact position
within the delta Scuti instability strip.





       D.W. COATES, L. HALPRIN, T.T. MOON and K. THOMPSON

          Department of Physics, Monash University

            Clayton, Victoria 3168, Australia





References:

Breger, M., 1979, Publs. astr. Soc. Pacif., 91, 5-26. [BIBCODE 1979PASP...91....5B ]
McInally, C.J. and Austin, R.R.D., 1978, Mon. Not. R. astr. Soc. 184, 885-891. [BIBCODE 1978MNRAS.184..885M ]