Variable stars in the field of the old open cluster NGC 188

We report the results of a new time-series charge-coupled device (CCD) photometry survey of variable stars in the field of the old open cluster NGC 188. In a 1degrees field covering the cluster, a total number of 27 variable stars, including eight new variables, were detected during this survey. The periods, the classifications as well as the membership of the newly discovered variables are discussed. Among those, two are definite member stars and one is a probable member star of the cluster. They are all W UMa binaries. Thus the total number of W UMa stars belonging to the cluster is increased to 10. A peculiar variable (V26 = GSC 4619-450) is found from the field. It is very likely a new large-amplitude beta Cephei-type variable with a pulsating period of about 0.1331 d. Revised basic data are given for the known variables in the programme field. The period of V8 is refined as 5.2096 d. The variability of V11 is confirmed by this observation. It is suggested to be a very probable FK Com-type variable with a period of about 1.2433 d. Finally, the relative frequency of the occurrences of W UMa stars and the distribution in the cluster are discussed. The result supports the hypothesis of mass segregation in NGC 188.