  This paper is the first of a series addressed to the investigation of galaxy
formation/evolution in small scale systems of galaxies (SSSGs) which are
located in low density cosmic environments. Our algorithm for SSSG selection
includes galaxy systems of 2 or more galaxies lying within 1000 km/s and a 200
h_{100}^{-1} kpc radius volume. We present the analysis of the photometric and
spectroscopic properties of 19 member galaxies belonging to a sample of 11
SSSGs.
  In the $\mu_e - r_e$ plane, early-type members may be considered "ordinary",
not "bright" galaxies in the definition given by Capaccioli et al.(1992) with a
significant fraction of galaxies having a disk or disky isophotes. We do not
detect fine structure and signatures of recent interaction events in the
early-type galaxy population, a picture also confirmed by the spectroscopy.
  At odd, there are several spiral members with open arm configurations as
expected in interacting systems. At the same time, emission lines in the
spectra of spiral members fall in the HII regions regime defined with
diagnostic diagrams (Veilleux & Osterbrock 1987). None of the objects displays
unambiguous indication of nuclear activity, although fourspiral nuclei could be
ascribed to the class of Seyferts. The star formation rate seems enhanced over
the average expected in spiral galaxies only for poorer SSSGs in particular
pairs (<50 solar masses per year) but without being in the range of starburst
systems.

Astrophysics