COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1694 Konkoly Observatory Budapest 1979 October 23 PHOTOELECTRIC TIMES OF MINIMA FOR SELECTED ECLIPSING BINARIES AND UPDATED EPHEMERIDES The following times of minima are derived from photoelectric V-filter observations during August-September of 1979 with the 40-cm reflector of the University of Montana. The observing procedure was identical to that described in IBVS No. 1478 (Margrave et al., 1978). Each time of primary minimum in Table 1 was determined by the least-squares fitting of a parabola to the observations. Table 1 lists the heliocentric Julian Date for each primary minimum observed, the epoch number E, the O-C value, and N, the number of observations used in the determination (each observation is the average of three 10-second integrations). The ephemerides used to calculate the O-C values are given in Table 2. Table 1. Heliocentric Times of Primary Minima Star Hel. JD - 2,440,000 E O-C N KO Aql 4135.7568 785 +0.0218d 36 RZ Cas 4096.7410 11,893 -0.0016 33 4121.8392 11,919 -0.0036 52 4127.8151 11,924 -0.0039 45 TV Cas 4094.9219 580 -0.0164 53 4114.8601 591 -0.0169 70 TW Cas 4110.8729 1,472 -0.0109 32 4123.7296 1,481 -0.0092 32 4130.8694 1,486 -0.0110 42 DO Cas 4095.7981 14,853 -0.0026 66 XX Cep 4133.9256 2,495 +0.0162 52 AT Peg 4089.8201 3,186 -0.0534 76 4128.7853 3,220 -0.0558 51 4136.8079 3,227 -0.0559 32 Table 2. Ephemerides for Program Stars Star Hel. JD Period (days) Source KO Aql 2,441,887.4714 2.86403 SAC 49 RZ Cas 2,429,875.6902 1.1952473 Herczeg and Friboes-Conde TV Cas 2,443,043.6265 1.8126066 SAC 50 TW Cas 2,442,008.3850 1.428328 SAC 50 DO Cas 2,433,926.4573 0.68466595 SAC 50 XX Cep 2,438,302.3209 2.33731 SAC 50 AT Peg 2,440,438.383 1.146105 SAC 50 The residual of KO Aql has increased steadily since August 1975. A linear fit to the epoch given in SAC 50, the IBVS No. 1478 minimum, and that of this note yields Hel. JD (Min) = 2,441,887.4733 + 2.864055d*E which fits these minima with residuals of -0.0019d, +0.0001d, and +0.0003d, respectively, Thus it appears that there has been an increase of 2.5d x 10^-5 in the period of KO Aql since 1973. The residual behavior of RZ Cas will be discussed elsewhere, insofar as it is linked to the question of sudden period changes versus light travel time effects (Doolittle, 1976). The provisional new ephemeris for TV Cas given by Margrave (1979) has been revised to include the minima of this note, with the following result: Hel. JD (Min) = 2,441,595.3598 + 1.8125898d*E. This ephemeris gives residuals less than +-0.0015d (except for one in 1978 of -0.0035d) for observations back to 1972. The above result reflects a decrease by 1.68d x 10^-5, or 1.45 seconds, relative to the period given in SAC 50. The residuals for TW Cas have generally become more negative since 1975, with primary eclipses now occurring about 15 minutes early. The epoch in SAC 50, the minima of IBVS Nos. 1478 and 1631, and those of this note yield the following linear ephemeris: Hel. JD (Min) = 2,442,008.3856 + 1.4283216d*E. The mean residual for the least-squares fit is 0.0026d. The new period reflects a decrease of 6.4d x 10^-6 with respect to the SAC 50 value. The residual for XX Cep given here continues the trend to later eclipse occurrences evident in 1975 (Margrave et al., 1978). Increasing the SAC 50 period to 2.3373158d reduces the residual of this note to +0.0018d and may perhaps be used for more accurate prediction over the next couple of years. The situation of AT Peg presently appears to involve a continuous period decrease at the rather large rate of 19.3 seconds per century, with primary minima being predictable by the following revised quadratic ephemeris: Hel. JD (Min) = 2,440,407.4368 + 1.14611077d*E - 7.0158 x 10^-9*E^2. This ephemeris fits the observations from 1969 to the present with a mean residual of 0.0026d. The epoch number for the last minimum of AT Peg in Table 1 is E = 3254 for the quadratic ephemeris. Continued timing of this system is necessary to determine the duration of the present stage of continuous period decrease. T. E. MARGRAVE Blue Mountain Observatory University of Montana Missoula, Montana 59812 USA References: Doolittle, J.: Master's Thesis, Univ. of Montana, 1976. Ebersberger, J., Pohl, E., and Kizilirmak, A.: IBVS No. 1449, 1978. Herczeg, T. and Friboes-Conde, H.: Astr. and Ap., 30, 259, 1974. [BIBCODE 1974A&A....30..259H ] Kizilirmak, A. and Pohl, E.: IBVS No. 937, 1974. Kordylewski, K.: Supplemento ad Annuario Cracoviense No. 49, Krakow 1978. Margrave, T. et al.: IBVS No. 1478, 1978. Margrave, T.: IBVS No. 1631, 1979. Pohl, E. and Kizilirmak, A.: IBVS No. 1163, 1976. Rudnicki, K.: Supplemento ad Annuario Cracoviense No. 50, Krakow 1978. [BIBCODE 1978raok.book.....R ]