COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2707 Konkoly Observatory Budapest 15 April 1985 HU ISSN 0374 - 0676 NEW TIMES OF MINIMA, AND A RECENT PERIOD INCREASE, IN U CEPHEI The very considerable data on period changes in U Cep was last summarized by Olson et al. (1981). Since then, Faulkner and Kaitchuck (1983) have determined three more times of minimum. We now give eight additional photoelectric times, determined with the 0.4-m and 1.0-m reflectors at Mount Laguna Observatory. The same uvby filter set was used for all observations, and I observations were also made with the 1.0-m telescope. Focal-plane apertures were either 20 or 25 arc-sec, and excluded both nearby visual companions (ADS 830 BC). Times of minima were determined using the Kwee, Van Woerden method, as programmed by R. C. Crawford. TABLE I NEW PHOTOELECTRIC TIMES OF MINIMUM LIGHT JD (hel) E O-C d Observer -2440000. (days) (days) 4633.8455 2544 +0.0472 0.082 O 4658.7768 2554 0.0481 0.074 H 4840.7686 2627 0.0479 0.082 H 4850.7413 2631 0.0484 0.081 H 5179.8252 2763 0.0509 0.081 H 5558.7746 2915 0.0581 0.089 P 5720.8302 2980 0.0660 0.066 1 Hu 6049.9151 3112 0.0695 0.090 O 1 Disturbed eclipse. [FIGURE 1] Figure 1 - Observed minus calculated times of minimum for U Cephei over the past six years. Large circles are new observations. Filled circles are from undisturbed eclipses, and open circles for disturbed eclipses (for which d <~ 0.075 day). TABLE II SUMMARY OF RECENT ABRUPT PERIOD CHANGES IN U CEPHEI Cycle Count of Year DeltaP/P Change 1315 1972 -2.5 x 10^-5 1599 1974 +1.0 x 10^-5 2284 1979 -0.8 x 10^-5 2750 1982 +1.9 x 10^-5 As noted by Crawford and Olson (1979), we assume that only those eclipses with apparent totality, d, >~ 0.075 day were undisturbed by circumstellar light, and therefore gave true times of conjunction. New times of minimum are listed in Table I, and all but one are undisturbed. We continue to calculate cycle count and (O-C) from the ephemeris JD(hel), I = 2438291.5020 + 2.493 0410E. Fig. 1 is a plot of residuals from this ephemeris, and includes previously published data back to the last period decrease. An 'abrupt' period increase occurred near E ~ 2750. Adding the four new observations before this increase to the eight undisturbed times since the last period decrease gives a refined ephemeris for the interval E = 2335-2631: JD(hel),I = 2438291.5324 +/- 0.0005 + (2.4930478+/- 0.0000014) E. Combining our three undisturbed minima after the increase with those of Faulkner and Kaitchuck (1983) gives the post-period increase ephemeris: JD(hel),I = 2438291.4048 +/- 0.0059 + (2.4930945 +/- 0.0000015) E. Recent period changes in U Cep are summarized in Table II. Both Fig. 1 and the corresponding figure in Olson et al. (1981) support the interpretation that abrupt period changes interrupt relative long intervals (a few years) during which the orbital period is sensibly constant. The abrupt period changes are of alternating sign. Though in the long run the period of U Cep has been increasing, there has obviously been a slight decrease over the interval covered in Table II. Disturbed eclipses continue to be spuriously late, as described by Crawford and Olson (1979). Finally, there continues to be no evidence whatsoever that these abrupt period changes have anything to do with sporadic episodes of mass transfer in U Cep. One of us (E.C.O.) thanks the U. S. National Science Foundation for continuing support of research on interacting binaries. We thank the Director and staff of Mount Laguna Observatory for observing time and assistance. EDWARD C. OLSON Department of Astronomy, University of Illinois 1011 W. Springfield, Urbana IL 61801-3000 USA JEFFREY P. HICKEY, LINDA HUMES, VICTORIA PAYLOR Department of Astronomy San Diego State University San Diego, CA 92182-0334 USA References: Crawford, R. C., and Olson, E. C. 1979. Publ. Astron. Soc. Pacific 91, 111. [BIBCODE 1979PASP...91..111C ] Faulkner, D., and Kaitchuck, R. 1983. Inf. Bull. Var. Stars, No. 2321. Olson, E. C., Crawford, R. C., Hall, D. S., Louth, H., Markworth, N. L., and Piirola, V. Publ. Astron. Soc. Pacific 93, 464. [BIBCODE 1981PASP...93..464O ]