1 Introduction

Star cluster pairs are common objects in the Magellanic Clouds and it is important to understand their formation and evolution processes. A list of 30 cluster pairs in the SMC was first presented by Hatzidimitriou & Bhatia (1990). Bica & Schmitt (1995, hereafter BS95) revised previous data on SMC extended objects (star clusters, associations and emission nebulae) and identified new ones using Sky Survey ESO/SERC R and J films. They presented a list of 40 pairs and 2 triple star clusters. Pietrzynski & Udalski (1999a) reported 23 pairs and 4 triplets derived from Pietrzynski et al.'s (1998) star cluster catalogue in the OGLE survey area. Bica & Dutra (2000) updated BS95's catalogue considering the entries in Pietrzynski et al.'s catalogue. Bica & Dutra (2000) indicated 75 pairs and multiplets comprising 176 individual objects.

In recent years there has been growing evidence of interacting star clusters in the Magellanic Clouds, especially in the LMC. Bhatia & Hatzidimitriou (1988) concluded that more than 50% of LMC pairs must be physical systems. Bhatia & McGillivray (1988) found evidence that NGC 2214 is a merging binary star cluster, based on the presence of a flattened core and an extended halo. Indeed Sagar et al. (1991) detected two turnoff points revealing the presence of two interacting clusters. Bica et al. (1992) studied cluster pairs and multiplets in the LMC bar by means of integrated colours and found systems which resulted coeval and some with age differences. Vallenari et al. (1998) confirmed such scenarios by means of colour-magnitude diagrams. They also presented isophotal contours indicating physical interaction. Several other studies have found binarity evidence in LMC cluster pairs (e.g. Kontizas et al. 1993; Dieball & Grebel 2000a, 2000b).

Bhatia et al. (1991) presented a photographic atlas of binary star cluster candidates in the LMC. For the SMC no morphological atlas is available and isophotal maps are required to test possible physical interactions. Indeed comparisons of isophotes with isopleth maps of N-body simulations proved to be a useful tool (Rodrigues et al. 1994; de Oliveira et al. 1998, hereafter ODB98), since the simulations produce features such as bridges, common envelopes and extensions. The observational importance of tidal tails as interaction signatures was also indicated by Leon et al. (1999).

In this work we provide isophotal maps for SMC pairs and multiplets to study the following properties of these candidate physical systems: (i) angular distribution; (ii) projected centre-to-centre separation of members; (iii) isophotal structures using the Digitized Sky Survey^* (hereafter DSS); (iv) ages derived by means of isochrone fitting, when possible. We discuss candidate physical systems and infer a scenario for their formation and evolution.

In Sect. 2 we gather the objects providing coordinates, sizes, centre-to-centre separations and other details for the SMC pairs and multiplets. In Sect. 3 we provide the isophotal contour atlas together with classifications of interaction features whenever present. A preliminary version of the present isophotal SMC atlas together with one for the LMC was given in de Oliveira (1996). In Sect. 4 we derive cluster ages by means of Colour-Magnitude diagrams extracted from the OGLE-II BVI photometric survey (Udalski et al. 1998). In Sect. 5 we discuss the properties of the systems and the possible scenarios for their origin and evolution. Finally, concluding remarks are given in Sect. 6.