COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1894 Konkoly Observatory Budapest 1980 December 19 HU ISSN 0374-0676 ON THE INTERCONNECTION OF THE PULSATIONAL AND ORBITAL PERIOD VALUES FOR BINARY SYSTEMS We pay attention to the existence of the unusual "synchronization" between pulsational and orbital motion in the close eclipsing and spectroscopic binaries containing pulsating components of different types: the orbital period contains almost an integer of pulsation cycles. Earlier several authors have already noticed the same effect for individual stars: Delta Scuti star AB Cas (O.A. Chekanikhina, M.S. Frolov, B.N. Irkaev, Inf. Bull. var. Stars, No. 1382, 1978), RR Lyrae stars V 80 in the dwarf galaxy Ursa Minor and RW Ari (K.A. Sidorov, Var. Stars 20, 557, 1978). Probably RW Ari is single RR Lyrae star according to recent observations. We have prepared the list of known Delta Scuti, Beta Canis Majoris and Cepheid pulsating stars in the short period eclipsing or spectroscopic binary systems. This list contains 27 objects with both orbital and pulsation periods known (in the case of multiperiodicity we prefer the period corresponding to the highest light amplitude). For 16 of these having the error of the ratio N=P_orb/P_puls not exceeded +-0.05 we have calculated the value Delta=|P_orb/P_puls - A|, where A is the nearest integer. Delta is the measure of deviation rate from the case of the exact synchronization (Delta=0). It stands to reason that the error of the ratio P_orb/P_puls depends not only on the period errors but also on the ratio value itself. Therefore one can say nothing on this effect in the case of wide pairs. The data on 16 binaries with pulsating components are given in Table I; eclipsing systems are denoted by asterisks. V 80 in UMi is the unique really eclipsing RR Lyrae star known to-day, which has been discovered by P.N. Kholopov (Var. Stars 18, 117, 1971). EN Lac is the first known to-day eclipsing system with the Beta CMa pulsating star. Both components of the spectroscopic binary Delta Del are Delta Scuti variables. Table No. Name Type N=P_orb/P_puls Delta=|(P_orb/P_puls)-A| 1* V 80(UMi) RR 4.15 0.15 2 FF Aql Cep 320.96 0.04 3 BL Tel Cep? 11.95 0.05 4 KW Aur Delta Sct 43.01 0.01 5*, Y Cam Delta Sct 49.74 0.26 6 Delta Del Delta Sct 299.38 0.38 261.82 0.18 7 V 644 Her Delta Sct 102.98 0.02 8* AI Hya Delta Sct 60.06 0.06 9 BS 8210 Delta Sct 49.75 0.25 10 Nu Cen Beta CMa 15.00 0.00 11 Beta Cep Beta CMa 57.18 0.18 12 V 600 Her Beta CMa 25.27 0.27 13* EN Lac Beta CMa 71.51 0.49 14 Psi Ori Beta CMa? 8.20 0.20 15 Gamma Peg Beta CMa 45.01 0.01 16 Alpha Vir Beta CMa 23.10 0.10 [FIGURE 1] In Fig. 1 we show the distribution in number versus the value Delta for these objects. One can see that the case of the strict synchronization (Delta=0) is the most probable state for these systems. Maybe our result on Y Cam, one of the eclipsing binaries with a Delta Sct pulsating component, is the evidence of the unusual pulsation in the presence of the close companion. We have observed this object photoelectrically during three nights JD 2444222-4 by using the 48-cm reflector of the Alma-Ata High Altitude station of the Sternberg Astronomical Institute. We have confirmed the results which had been received earlier by P. Broglia and.F. Marin (Astron. Astrophys., 34, 89, 1974) on Delta Scuti nature of the brighter component of this Algol system (Fig. 2). The light amplitude is strongly changing during our short time observing interval [FIGURE 2] from 0.05m to about 0.01m in B light. The period also varies from 0.056d to 0.073d. We have determined the average period for the pulsating component 0.066458 on the basis of long series of observations published for Y Cam (P. Broglia, P. Conconi, Milano-Merate Publ. No. 27, 1973) by using S. Yu. Shugarov's programme for period determination. The observations during minima were excluded. We were surprised that it was possible to obtain the mean curve based on only single period during 13000 pulsation cycles for such a "semiregular" star. This mean light curve is shown in Fig. 3. [FIGURE 3] Maybe, just the presence of the close companion makes the star pulsate much more regularly than in the single case. What is the reason of the "synchronization" between the pulsation of the component and the orbital movement in shortperiodic binaries? Maybe, this is an effect of the tidal action of the close companion on the pulsational process in accordance with the well known Fitch's hypothesis. Maybe, however, this is the coincidence of the P_puls and not the P_orb but the axial rotation period P_rot of the same pulsating star. Indeed, P_orb=P_rot for many binaries. If so, the binarity is only the good tool for discovering the coincidence because P_rot is unknown for the single pulsator due to the "sin i" factor. In the light of the second hypothesis one can suppose that the range of dispersion seen in Fig. 1 must be smaller in reality: for several systems possibly P_orb non equal P_rot. Just in these cases the large values of Delta (poor synchronization) can occur. It would be interesting to test this; maybe Delta Del is one of such systems. M.S. FROLOV, E.N. PASTUKHOVA A.V. MIRONOV, V.G. MOSHKALEV Astronomical Council of the Sternberg Astronomical Institute USSR Academy of Sciences USSR