1 Introduction

Two-dimensional broad-band photometry has been systematically applied to the study of binary galaxy structure and dynamics, only in the past two decades. The mid to late 70's saw an astronomical debate that led to the recognition that gravitational interaction is an important factor in galactic evolution affecting directly properties such as size, morphological type, luminosity, star formation rate, and mass distribution (Sulentic 1976; Larson & Tinsley 1978; Stocke 1978). According to current popular models of galaxy formation, galaxies are assembled through a hierarchical process of mass aggregation, dominated either by mergers (Kauffmann et al. 1993; Baugh et al. 1996) or by gas accretion (Avila-Reese et al. 1998; Avila-Reese & Firmani 2000). In the light of these models, the influence of environment factors and interaction phenomena in the shaping and star formation of the disks is natural, at least for a fraction of the present-day galaxy population. Examples are galaxy harassment in clusters (Moore et al. 1996), tidal stirring of dwarf irregulars near giant galaxies (Mayer et al. 2001), tidally induced star formation (Lacey & Silk 1991; Kauffmann et al. 2001). See also Dultzin-Hacyan (1997) for a non-biased review.

For binary galaxies, current ideas suggest that most physical pairs are morphologically concordant, that is, with components showing similar initial star formation and angular momentum properties. Evidence in favour of these ideas come from the colour correlations (Holmberg effect) observed for components of pairs, although very few photometric data supporting this correlation exists at present. The large number of (S+S) pairs in The Catalogue of Isolated Pairs of Galaxies in the Northern Hemisphere (KPG, Karachentsev 1972) means that for a flux-limited sample ( m_zw = 15.7), almost six out of every ten pairs are of the (S+S) type, suggesting that a considerable number of them must be physical binaries.

Redshift information, available for the whole (S+S) sample, suggests that most are likely to be physically proximate. Digital Sky Survey images show that most have visible signs of disturbance; bridges, tails, common envelopes and distortions that are regarded as evidence for gravitational interaction. In addition, statistical studies indicate that a high fraction (${\sim} 65$%) show an enhancement in the optical and FIR emission (Xu & Sulentic 1991; Hernández-Toledo et al. 1999). This enhancement is interpreted as a by-product of interaction-induced star formation activity in physical binaries.

One of the most important lessons that emerges from statistical studies of interacting galaxies is that if we want to have a better understanding of the connection between interactions, photometric structural parameters and optical morphology, it is necessary to obtain accurate photometry for complete and homogeneous interacting galaxy samples. The results can then be interpreted by applying similar methods to well-matched comparison samples (isolated, presumably undisturbed galaxies). Some efforts in this direction are Bergvall & Johansson (1995), Reduzzi & Rampazzo (1996), Márquez & Moles (1996), Laurikainen et al. (1998), Jansen et al. (2000), Márquez & Moles (1999), and de Jong & van der Kruit (1994).

Our main goal is to obtain an homogeneous set of broad-band observations for most (if not all) of the (S+S) pairs in the Karachentsev Catalogue. The relative simplicity, compared to groups and clusters along with the size, brightness limit and morphological diversity, offer us a unique opportunity to realize accurate photometric observations for a statistically significant number of pairs where a less confused interpretation of the photometric properties of disk galaxies at different stages of interactions (and its relationship to optical morphology) is possible.

We present in this first paper of a series, our photometric data with emphasis on the morphological properties in a subset of 33 (S+S) pairs. The structure of the paper is as follows. Section 2 summarizes some limitations and biases in the (S+S) sample which are relevant to our photometric study. Section 3 presents the observations, reduction techniques and a comparison of our estimated total magnitudes against those in the literature. A discussion on the related errors is also included. Section 4 shows a discussion based on the estimated colour indices, observed optical morphologies and (colour and surface brightness) profiles for each component galaxy. The systematics of morphological distortions induced by the interactions are commented in the light of current models. Section 5 is a summary of the conclusions achieved. Finally, an Appendix is devoted to the presentation of magnitudes at three different concentric circular apertures.