7 Conclusion

We have imaged an ${\sim}13\times 13$ arcmin² region of the M 31 disk for about 4 years from 1998 to 2001 in R and I passbands. Using 141 nights of observation spanning a period of $\sim$1200 days, we have detected 26 Cepheids among which variability in 2 stars has been found for the first time. We have established the Cepheid nature of 11 variables in the sample, while we confirm the period and nature of variability in the other 13 stars. The long duration data enabled us to obtain accurate period and mean magnitude of the Cepheids. The period obtained for these Cepheids ranges from $\sim$7.5 to 56 days. They range in age from 22 Myrs to 75 Myrs and occupy a strip where the early type supergiants too are located. Using the period-luminosity relation of the Cepheids, we derive a distance of 790$~\pm~$45 kpc for M 31. There could be additional uncertainty in the distance determination due to blending effect in Cepheids as well as variable extinction within the observed region.

In addition to Cepheids, we also detect 333 variable stars. Out of them, 115 are periodic variables, 2 suspected eclipsing binaries and remaining 216 stars appear to be irregular variables.

We have thus demonstrated once more that meter class telescopes like ours can play an important role in the study of variable stars provided a large amount of telescope observing time is made available. These data are also valuable for the search of gravitational microlensing events towards M 31. We are in the process of identifying lensing candidates in our target field and their detailed study shall be presented in a forthcoming publication.

Acknowledgements

We would like to thank the referee Dr. B. J. Mochejska for useful remarks on the manuscript. We are also thankful to Dr. Vijay Mohan for his helpful advice during observations and Drs. Jean Kaplan, Alan Bouque and Vincenzo Cardone for useful discussions during the course of the work. This study is a part of the project 2404-3 supported by Indo-French center for the Promotion of Advance Research, New Delhi.