1 Introduction

In recent years, the Andromeda galaxy (M 31) has been a target of searches for gravitational microlensing events (Crotts & Tomaney 1996; Ansari et al. 1997). To detect lensing events, continuous observations are needed for a long duration though for a short period of time each night. Such observations are, therefore, very useful to search for variable stars (e.g. Cepheids, Miras) also. The catalogues of variable stars compiled from such a monitoring programme are generally complete within the flux limitation, because of the continuous observation of the same field over a long period. Various groups (e.g. MACHO, EROS, OGLE) dedicated to search for microlensing events in the Galactic Bulge and Magellanic Clouds have already identified a large number of variable stars as a bi-product (Welch et al. 1997; Udalski et al. 1999a) and the catalogues of Cepheid variables brought out by these projects (Beaulieu et al. 1995; Udalski et al. 1999b) have given insight into the pulsation properties of Cepheids.

In collaboration with the AGAPE (Andromeda Gravitational Amplification Pixel Experiment) group, we started Cousins R and I photometric observations of M 31 in 1998 with an aim to search for microlensing events. Based on the observations of first 2 years, Joshi et al. (2001, hereafter referred as Paper I) reported 8 variables, 7 of them were suspected as Cepheids and one Mira-like variable. In this paper, we extended our analysis to detect the variable stars using four years of observations. Details of our observations are given in next section while Sect. 3 outlines the detailed photometry of our data. The detection of variable stars is described in Sect. 4 which leads to discovery of some new Cepheids. The catalogue of Cepheids, their period-luminosity relation, colour-magnitude diagram along with a brief discussion of some of the Cepheids are presented in Sect. 5. Other type of variables along with discussion and conclusion are given in the remaining part of the paper.

 

 

Table 1: Characteristic parameters of the two CCDs used for observations.

Size of CCD	Field	Quantum Efficiency		Gain	Readout Noise	Pixel size
(pixel2)	(arcmin2)	R(%)	I(%)	(e-/ADU)	(e-)	(arcsec)
$1024 \times 1024$	${\sim}6 \times 6$	35	31	2.96	4.1	$\sim$0.37
$2048 \times 2048$	${\sim}13\times 13$	74	61	10.00	5.3	$\sim$0.37