COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2084 Konkoly Observatory Budapest 1982 February 10 HU ISSN 0374-0676 ON THE RADIAL PULSATIONS OF THE DELTA SCUTI STARS SIGMA OCTANTIS AND B OCTANTIS The southern Delta Scuti variables sigma Octantis (=HR 7228= HD 177482) and B Octantis (= HR 8294= HD 206553) were discovered by McInally and Austin (1978). Recently these stars have been again observed by Coates et al. (1981), who plan in the future to measure the colour indices of B Octantis in order to locate its position within the instability strip. In the present note, on the basis of our results (Tsvetkov, 1981 a,b) we estimate the expected locations of these two variables within the instability strip and discuss the possible pulsation modes of B Octantis. 1. sigma Octantis. Coates et al. (1981) determined a single oscillation period P = 0.097 day with a visual light amplitude DeltaV = 0.025 mag. They indicate that sigma Octantis pulsates probably in the fundamental mode only. In such a case, from our semiempirical period - luminosity relation for this mode one may estimate its absolute magnitude: M_bol ~= M_v = (1.60 +- 0.23) mag. sigma Octantis is not likely situated near the observed blue edge of the instability strip, where the Delta Scuti stars pulsate in overtones. At the above evaluated luminosity, its effective temperature within the observed instability strip (Breger, 1979) is expected to be in the range 3.83 <= log T_e <= 3.90. 2. B Octantis. In this case the problem is more complicated. Coates et al. (1981) found two oscillations: one with a period P = 0.063 day and an amplitude DeltaV = 0.010 mag, and an other with P = 0.143 day and DeltaV = 0.014 mag. The authors note that the observed period ratio of 0.44 cannot be obtained from oscillations in the four lowest modes. It is known that the radial pulsations of Delta Scuti stars lead to the following mean period ratios: ~= 0.76, ~= 0.62, ~= 0.52. We suggest that the observed period ratio for B Octantis may be explained, if one accepts that this star pulsates in both fundamental mode and fourth (or perhaps fifth) overtone. Indeed, if the mean value of the pulsation "constant" for the fourth overtone is Q_4 = 0.015 day, then with Q_0 ~= 0.033 day one derives a mean period ratio ~= Q_4/Q_0 ~= 0.45 in a good agreement with the observed period ratio ratio for B Octantis. Assuming that the observed period of 0.143 day may be attributed to the fundamental mode, one may estimate the absolute magnitude of this star: M_bol M_v (1.12 +- 0.23) mag. At such a luminosity, pulsations in the fundamental mode within the observed instability strip are possible for 3.82 <= log T_e <= 3.90. In conclusion we notice that there is an evidence for oscillation in a high mode for other variables too (Tsvetkov, 1981 b): V 1004 Ori (= HR 2100 = HD 40372) and BN Cnc (= HD 73763). For these two stars, in particular, one obtains an "observed" pulsation "constant" of 0.015 day, which indicates a mode higher than the third overtone (Q_3 = 0.017 days). Hence the observations give evidences that some Delta Scuti variables may perform radial oscillations in the fourth (or even fifth) overtone. TS.G. TSVETKOV Department of Astronomy University of Sofia Anton Ivanov Street 5 1126 Sofia, Bulgaria References: Breger, M., 1979, Publ.Astr.Soc.Pacif., 91, 5. [BIBCODE 1979PASP...91....5B ] Coates, D.W., Halprin, L., Moon, T.T. and Thompson, K., 1981, Inform.Bull.Var.Stars, No. 2047 McInally, C.J. and Austin, R.R.D., 1978, Mon.Not.R.Astr.Soc., 184, 885. [BIBCODE 1978MNRAS.184..885M ] Tsvetkov, Ts.G., 1981 a - "Theoretical Blue Edges of the Delta Scuti Instability Strip", Soviet Astron. Zh. (in press). Tsvetkov, Ts.G., 1981 b - "Radial Pulsation Modes and Possible Heterogeneity of Delta Scuti Stars", Soviet Astron. Zh. (in press).