COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3669 Konkoly Observatory Budapest 7 October 1991 HU ISSN 0374 - 0676 PERIOD CHANGES OF THE BRIGHT RR LYRAE STARS SU Dra AND VY Ser Alerted by visual observers of the Berliner Arbeitsgemeinschaft für Veranderliche Sterne (BAV) we did some photoelectric photometry on the bright RRab stars SU Draconis and VY Serpentis in search of suspected changes in the primary periods. The telescope used was a 0.34 m Cassegrain, equipped with a 1P21 phototube. All measurements were done in V colour. SU Dra was discussed in detail by Olah and Szeidl (10), who detected no major period changes, apart from some very small fluctuations, and published the elements: JD hel. Max. = 2420605.762 + 0.66041890 * E (I) Table 1 shows a collection of photoelectric maxima of SU Dra obtained since JD 2442000. O-C residuals (I) are given for the elements of Olah and Szeidl (10) and (II) for elements calculated by us on the method of least squares, namely: JD hel. Max. = 2448024.4013 + 0.66042153 * E (II) +-17 +-28 Table 1: Photoelectric maxima of SU Draconis. JD hel. O-C (I) O-C (II) Observer Reference 2442403.5545 +.0063 +.0008 Olah and Szeidl (10) 42415.4444 +.0086 +.0032 Olah and Szeidl (10) 42454.4077 +.0072 +.0016 Olah and Szeidl (10) 42948.3975 +.0037 -.0039 Olah and Szeidl (10) 43204.6427 +.0064 -.0023 Olah and Szeidl (10) 45054.4846 +.0149 -.0011 Ag Braune and Mundry (1) 46833.6643 +.0261 +.0030 Liu and Janes (8) 48024.4036 +.0301 +.0023 Gr this paper 48127.4237 +.0249 -.0034 Gz/Wu this paper The newly established period clearly differs from Olah and Szeidl (10) and, compared with earlier observations, which are mentioned in a more detailed paper by Wunder (13), a major period increase seems to have occurred at around JD 2441000. The fourth edition of the GCVS (6) already lists a somewhat larger period (P=0.66092001) than Olah and Szeidl (10) found, which nevertheless fails to represent recent observations. Regarding VY Ser Table 2 lists all available photoelectric maxima of this RRab star. Table 2: Photoelectric maxima of VY Serpentis. JD hel. O-C (I) O-C (II) Observer Reference 2437411.531 -.005 +.007 Tremko (12) 38466.958 -.009 +-.000 Fitch et al.(4) 39213.901 -.008 -.002 Fitch et al.(4) 39615.933 -.011 -.006 Stepien (11) 41443.309 -.001 -.002 Lub (9) 44738.869 +.016 +.005 Carney and Latham (2) 44764.566 +.006 -.006 Fernley et al.(3) 47307.4768 +.028 +.008 Ls/Sk/Wu this paper 47653.798 +.014 -.007 Fernley et al.(3) 48356.4777 +.026 +.002 Wu this paper Ephemeris (I) refers to the elements of the fourth edition of the GCVS (7) : JD hel. Max. = 2431225.341 + 0.71409384 * E (I) while O-C residuals (II) are based on the elements derived by us: JD hel. Max. = 2448356.4756 + 0.71409615 * E (II) +-31 +-3 Jones et al. (5) and Fernley et al. (3) also published new elements for VY Ser. Both differ significantly from our elements because of too few observations. Including non-photoelectric maxima from JD 2415800 to JD 2437000, which are mentioned in a more detailed analysis by Wunder (13), it seems likely that the period variation is not due to an abrupt change, but is reflected by a continuous and slow increase of the period by 1.2 seconds per century. Also this is somewhat uncertain because of the high scatter of these observations, the process can be described by the formula: JD hel. Max. = 2438873.2763 + 0.71409494 * E + 1.38 * 10^-10 E^2 +-17 +-14 +-10 Abbreviations of the observer's names: Ag = F. Agerer Gz = M. Garzarolli Sk = S. Skaberna Gr = R. Gröbel Ls = G. Lichtschlag Wu = E. Wunder E. WUNDER Nürnberg Observatory Regiomontanusweg 1 8500 Nurnberg-20, F.R.G. References: (1) Braune, W., Mundry E., 1982, BAV-Mitteilungen, No. 34 (2) Carney, B.W., Latham, D.W., 1984, Astrophysical Journal 278, 241 [BIBCODE 1984ApJ...278..241C ] (3) Fernley, J.A. et al., 1990, Monthly Notices of the Royal Astr. Soc. 247, 287 [BIBCODE 1990MNRAS.247..287F ] (4) Fitch, W.S., Wisniewski, W.Z., Johnson, H.L., 1966, Comm. 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