1 Introduction

Star clusters of the Magellanic Clouds are, on average, less well studied than the bulk of the globular clusters of our Galaxy. At the same time, the rich populous clusters of the Magellanic Clouds, similar in the appearance to Galactic globulars, show a much wider range of ages, and therefore are especially interesting from the point of view of star formation and stellar evolution studies.

Among the populous star clusters in the Magellanic Clouds, of special interest are intermediate-age clusters with red horizontal branches: in our Galaxy, clusters with red horizontal branches are much older (approximately by ten billion years) and not sufficiently well-studied.

The Large Magellanic Cloud cluster, NGC 1978 ($\rm \alpha=5^h28^m44\hbox{$.\!\!^{\rm s}$}8$, $\delta=-66^\circ14'10''$, 2000.0) is a populous intermediate-age, red-horizontal-branch cluster immersed in a dense stellar field. Two detailed photometric studies have been devoted to this cluster. Olszewski (1984) published photographic BV photometry of cluster and field stars reaching slightly below the cluster main-sequence turnoff. The only multicolor CCD photometry of the cluster and surrounding field was carried out by Bomans et al. (1995). (The corresponding BV data for individual stars can be retrieved from Strasbourg Data Center as the electronic supplement to the paper of Will et al. 1995). In particular, these papers are in agreement in regard to estimates of the cluster's metallicity (-0.4 to -0.5 in [Fe/H]) and its age (2-2.5 Gyr). They also present detailed reviews of the earlier studies of the cluster.

We have obtained BVRI CCD photometry of 3077 stars in a $5'\times5'$field including NGC 1978. In this study, we concentrate primarily on the morphology of the cluster's brighter sequences and its possible variation in different parts of the cluster field.