Search for variable stars in two old open clusters: NGC 2506 and NGC 2420

We present results of photometric observations of two old open clusters: NG C 2506 and NGC 2420. V-band time-series and UBVI absolute CCD photometric o bservations were carried out to search for variable stars and to investigat e their physical properties. From the UBVI photometry of NGC 2506, we obtai n interstellar reddening of E(B - V) = 0.04 +/- 0.03 mag, distance modulus (V - M-v)(0) = 12.5 +/- 0.1 mag, and age log (t/yr) = 9.25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three delta Sct sta rs and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no gamma Dor star among main-sequence stars near FO in these two old open clusters, which is consistent with the suggestion that gamma Dor-type phenomenon occurs in stars younger than log (t/yr)= 8.4. On the other hand, it has been known that color-magnitude diag rams of these two clusters show well-established binary sequences, implying high incidence of binary systems: greater than or equal to 20% for NGC 250 6 and approximate to 50% for NGC 2420. However, only one eclipsing binary s tar was found in these two clusters.