1 Introduction

The analysis of the orbital period variations of eclipsing binaries is very useful for the study of many physical processes. It provides estimates of mass transfer and loss, angular momentum transfer and loss, stellar evolution, third body problem, etc. In this paper, the long-time variations in the orbital periods of some Algol-type binaries, UW Cyg, RX Hya, AK Ser, AC Tau, UW Vir and VV Vul, are analyzed. Then the mechanisms that could be at the origin of the changes in the orbital periods are discussed.

The light variation of RX Hya was discovered by Miss H.S. Leavitt (Pickering 1907). After its discovery, the system was observed visually by many authors (e.g., Zinner 1922; Maggini 1921; Dugan 1933 and others). The first radial-velocity curve for the single-line binary system was published by Struve (1946) and the spectroscopic elements were also determined by him. Vyas & Abhyankar (1989) obtained the first complete photoelectric U, B and V light curves and analyzed these observations with Wilson-Devinney (W-D) method. Their studies suggest that RX Hya is a semi-detached Algol type binary with the secondary filling its Roche Lobe. By combining the spectroscopic elements from Struve (1946), the absolute parameters of the system are determined.

According to General Catalogue of Variable Stars (Kholopov 1985), the other five systems, UW Cyg, AK Ser, AC Tau, UW Vir and VV Vul are also semi-detached binaries with orbital periods less than four days. The general properties of the five systems and those of RX Hya are shown in Table 1. Up to now, neither photoelectric or CCD light curves nor radial velocity curves of the stars have been published. Although ephemerides of these eclipsing binaries have been given by several authors (Mallama 1980; Wood & Forbes 1963; Whitney 1959), they are neglected for period studies.

 

 

Table 1: General properties of Six eclipsing binaries

Stars	Sp.	P	M1	R1	M2	R2	Ref.
UW Cyg	F0	3.45	1.71	2.28	0.59	2.79	(1)
RX Hya	F1+G5IV	2.28	2.72	2.17	0.66	2.93	(2)
AK Ser	A5	1.92	1.98	1.97	0.93	2.12	(1)
AC Tau	F0	2.04	1.45	2.30	0.99	2.90	(1)
UW Vir	A2+k3IV	1.81	1.77	1.52	0.43	2.08	(1)
VV Vul	A2/3V	3.41	-	-	-	-	(3)

References in Table 1:

(1) Brancewicz & Dworak (1980);

(2) Vyas & Abhyankar (1989);

(3) Kholopov (1985).