COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1529 Konkoly Observatory Budapest 1978 December 29 sigma^2 CrB: A NEW VARIABLE STAR, SHOWING POSSIBLE RS CVn-TYPE AND delta Sct-TYPE VARIABILITY The bright star sigma CrB = HR 6063 + 6064= ADS 9979AB was observed photoelectrically to search for variability. It appears as CSV 101569 in the (First) Catalogue of Stars Suspected of Variability. The brighter component (sigma^2 CrB = HR 6063= ADS 9979A) is a non-eclipsing SB2 RS CVn-type binary and we suspected the characteristic distortion wave may be present in its light curve. Tanner (1949) redetermined the spectroscopic orbit with the correct orbital period of 1.14d. According to Petrie (1950) the two stars in the SB2 system are comparable in luminosity, with Delta m = 0.2m. Young and Koniges (1977) found strong H and K emission from both. Spangler et al. (1977) have detected radio emission from sigma CrB. The spectral classification of sigma^2 CrB, apparently a composite of the SB2 system, appears in the literature as dF6 or F8. We compute phases with the ephemeris JD(hel.) = 2423869.105 + 1.139789d n of Tanner, where the initial epoch is his T_0, an epoch of maximum radial velocity for the more massive star. The accumulated effect of the +-0.000007d uncertainty in Tanner's period generates an uncertainty of +-0.1p in our computed phases. Altogether 62 differential magnitudes were obtained on nine different nights between JD 2443696.6 and 2443758.6. The telescope was a 12-inch Cassegrain reflector and an unrefrigerated 1P21 photomultiplier at -700 Volts was used with a filter chosen to approximate V of the UBV system. The diaphragm, about 1 arcminute in diameter, included sigma^1 CrB and also ADS 9979D (m_v ~12.5m) but not ADS 9979C (m_v ~10.0m). The comparison star was HR 6043= ADS 9958AB and the diaphragm included both components of that visual system also. The necessary corrections for differential atmospheric extinction were applied but were very small. There is indication of a sinusoidal variation with an amplitude of approximately DeltaV = 0.05m and a minimum at approximately 0.4p, although there is considerable scatter about such a sine wave, moreso than would be expected from observational uncertainty. On six of the nine nights observations were obtained in the following sequence: about 4 observations made within an interval of ~5 minutes followed by 4 more observations made ~1 hour later. The scatter within each group of 4 suggests an uncertainty of only about +-0.005m for the mean. (Comparable precision was characteristic of photometry of HR 7275 and HR 8575 which we obtained on most of those same nights). The difference between two means, however, ranged between 0.013m and 0.040m. Also we note that the maximum deviation of the means from the sine wave was 0.025m. We conclude that this short timescale variability, if real, could be described as resulting from a cyclical variation having a period of ~ 0.1d and a total amplitude of ~0.05m. The simplest explanation would be delta Scuti-type variability in one star of the SB2 system. The period is quite typical of known delta Scuti variables. If one removes the light of sigma^1 CrB (about 1m fainter in V than sigma^2 CrB) and also the light of the comparably bright companion star in the SB2 pair, one finds an intrinsic amplitude of 0.12m for the suspected delta Scuti-type variability. Although it is possible that the comparison star HR 6043 or sigma^1 CrB could be variable, neither is likely to be a delta Scuti variable. HR 6043 is spectral type gK2 and sigma^1 CrB is dG1 whereas sigma^2 CrB is F-type. Returning to the 1.14d sinusoidal variation, we feel this is most likely a result of the distortion wave characteristic of other RS CVn variables. Clearly this is not the ellipticity effect, because the light varies as a function of Theta, not 2 Theta. We can also show, by computing an upper limit, that this is not the differential reflection effect. A total mass of about 2.5 M_Sun for the SB2 system would imply i = 30d. We recall that the two stars are roughly comparable in luminosity. The maximum differential reflection effect would occur if one star had the smallest possible radius (main-sequence) and the other star had the largest possible radius (filling its Roche lone). With these assumptions, reflection would produce a variation of total amplitude DeltaV <=0.017m. The inclusion of sigma^1 CrB (about 1m fainter than sigma^2 CrB) in our photometry reduces this limit to DeltaV <=0.012m. This is considerably smaller than our observed amplitude of DeltaV=0.05m. The reflection effect should produce a minimum of light at one of the two conjunctions: either 0.25p or 0.75p based on Tanner's ephemeris. This is apparently not consistent with our minimum at 0.4p, although the +-0.1p uncertainty in our phases prevents a firm conclusion in this respect. The proposed delta Scuti variability would be easier to understand if one component of the SB2 pair were somewhat earlier than the composite type (which is F6 or F8). Likewise, the RS CVn-type distortion wave would be easier to understand if the other component were later than the composite type. The time is ripe for more observational work on sigma CrB, none of it difficult for such a bright star. (1) Photometry of sigma CrB should be repeated with continuous observation over intervals longer than the suspected 0.1p period of the suspected delta Scuti. variability, not only to confirm the reality and nature of that variation but also to remove that effect and thus define better the distortion wave. (2) A check star should be observed to exclude with certainty the possibility that our comparison star HD 6043 is variable. (3) A classification spectrum should be obtained at quadrature in order to determine separate spectral types for the two components of the SB2 system. (4) An up-to-date spectroscopic orbit should be obtained in order to remove the +-0.1p uncertainty in computed orbital phases and thereby make it possible for subsequent photometry to ascertain whether the distortion wave is migrating with respect to the orbital period. DAVID R. SKILLMAN DOUGLAS S. HALL + NASA Goddard Space Flight Copernicus Astronomical Center, Greenbelt, Maryland Center, 00-716 Warsaw 20771, U.S.A. Poland + On leave from Dyer Observatory, Vanderbilt University Nashville, Tennessee 37235, U.S.A. References: Petrie, R.M. 1950, P.D.A.O. 8, 319 [BIBCODE 1950PDAO....8..319P ] Spangler, S.R., Owen, F.N., and Hulse, R.A. 1977, A.J. 82, 989 [BIBCODE 1977AJ.....82..989S ] Tanner, R.W. 1949, P.D.A.O. 1, 473 [BIBCODE 1949PDDO....1..473T ] Young, A. and Koniges, A. 1977, Ap.J. 211, 473 [BIBCODE 1977ApJ...211..836Y ]