1 Introduction

According to present knowledge, the large scale structure of the Universe, as derived from recent redshift surveys, reveals large inhomogenities and a complicated void-supercluster network. This picture of the large scale structure of the Universe is mainly shown by the distribution of bright galaxies. But whether the giant luminous galaxies are fair tracers of the large scale structure and whether the voids are really empty has been subject of intense debate. According to the biased galaxy formation scenarios (i.e. Dekel & Silk [1986]) the dwarf galaxies should trace the underlying dark matter and are expected to fill the voids.

A search for faint galaxies in voids was accomplished by Hopp et al. ([1995]) and Kuhn et al. ([1997]), with the intention to overcome the limitations of previous surveys in magnitude, diameter and surface brightness. As a result, a sample of galaxies of late morphological type and of low surface brightness was identified, with several very isolated galaxies and galaxies populating the rims of the voids. Vennik et al. ([1996]) performed B and R surface photometry of a subsample of these galaxies and concluded that both isolated and non-isolated galaxies show similar photometric and structural characteristics.

There are also interesting results from the study of the spatial distribution of emission-line galaxies (ELGs) (Salzer [1989]; Weistrop et al. [1992], etc.). The ELGs are intrinsically small, very compact and with low luminosities. They look often almost stellar with no obvious underlying galaxy. Such galaxies can easily be missed by morphological surveys and, thus, are good candidates to fill up the voids. There have been even suggestions that a few ELGs have been found in the voids. Much effort has been concentrated on the study of the famous Bootes void, where several dozen of ELGs have been detected (Weistrop et al. [1995]; Szomoru et al. [1996]). The ELGs inside the Bootes void are nevertheless luminous galaxies, with strong star formation activity and not the faint objects predicted to be found in voids. A new sample of ELGs was identified on the IIIa-J objective prism plates towards nearby voids by Popescu et al. ([1996,1997,1998]). The objects were mainly selected by the presence of the high-excitation [O III]$\lambda$5007 emission line. Redshifts were measured for all the ELGs and their spatial distribution and isolation was evaluated from the cone diagrams together with the calculated nearest-neighbour distances (Popescu et al. [1997]). A subsample of 16 isolated galaxies was found, with the isolated galaxies distributed within the nearby voids.

For most of the identified ELGs CCD photometry was obtained in the Johnson B and Cousins R bands. Here we present the surface photometry of a subsample of 139 ELGs (and 2 non-ELGs) and we discuss their photometric properties. A more detailed analyse of the new data and a study of the environment influence on the photometric properties of the ELGs will make the object of a separate paper. In Sect. 2 the observations and reductions are described. Section 3 deals with the surface photometry algorithm; Sect. 4 evaluates the integral consistency of our results. Section 5 outlines the morphological classification and profile fitting. Some snap-shot statistics are fulfilled in Sect. 6 and Sect. 7 summarizes the obtained results.

A Hubble constant of H₀ = 75 km s^-1 Mpc^-1 is used throughout of this paper.