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Introduction

Since the pioneering work of Markarian ([1967]) objective prism surveys were recognized as a powerful tool to search for galaxies of various types of activity. As the result of nearly a dozen large objective prism surveys completed during the two last decades the main statistical properties were derived for various types of AGNs and for galaxies with enhanced current star formation.

While the selection of objects from objective prism spectra by their enhanced UV continuum was the primary criterion applied by Markarian ([1967]) and has proven to be especially efficient in searching for new Seyfert galaxies and QSOs, as well as for various types of star forming (SF) galaxies, the selection on the base of strong emission lines was shown to be also very efficient both in search for AGNs and galaxies with strong current star formation.

Several large samples of such galaxies have appeared as the results of University of Michigan (UM), Tololo and Calan-Tololo (Smith et al. [1995]; Salzer & MacAlpine [1988]; Salzer [1989b]; Terlevich et al. [1991]; Maza et al. [1991]), Case (Salzer et al. [1995]), Second Byurakan (SBS) (Izotov et al. [1993a]; Stepanian [1994]; Pustilnik et al. [1995]), and the Heidelberg void survey (Popescu et al. [1996], [1997], [1998]).

Of special interest are blue compact galaxies (BCGs). Because of their strong emission lines they are often classified as HII-galaxies as well. They are underluminous galaxies with an intense star formation burst taking place currently. BCGs are in general blue (B-V < 0.5), have small sizes (typically less than 10 kpc), low metallicity (Z < 1/3 $Z_\odot$) and are gas-rich (Thuan & Martin [1981]). They attracted much attention since their first description by Sargent & Searle ([1970]). Even earlier, these galaxies were recognized as objects which deserve attention in works by Haro ([1956]), Zwicky ([1964]) and Markarian ([1967]). As most of the BCGs have strong emission lines they can be classified as HII-galaxies as well (Veilleux & Osterbrock [1987]). In this and following papers we will use the term BCG for designaton of blue compact galaxies with HII-type emission-line spectra.

There are several important aspects in studies of BCGs. Searle & Sargent ([1972]) have noted that they can be truly young galaxies, forming their first stars. The outstanding examples of such objects are I Zw 18 (Sargent & Searle [1970]; Kunth et al. [1995]) and SBS 0335-052 (Izotov et al. [1990]; Izotov et al. [1997a]) with metallicities of 1/50 $Z_\odot$ and 1/41 $Z_\odot$, respectively. Another intriguing question is on possible evolutionary links of BCGs with other types of low-mass galaxies (dEs, dIs, LSBDs (see, e.g. Davies & Phillips [1989])). The spatial distribution of low-mass galaxies is still debated in connection with predictions for various models of galaxy formation (Bardeen et al. [1986]; Dekel & Silk [1986]). Despite of 10 years studies (Salzer [1989b]; Thuan et al. [1991]; Weinberg et al. [1991]; Rosenberg et al. [1994]; Pustilnik et al. [1995]; Popescu et al. [1997]) there is still no self-consistent picture. The strong emission lines in the spectra of BCGs allow to detect them easily far beyond the Local Supercluster. This advantage is very important for compiling large (covering over $\sim$ 1000 square degrees) and relatively deep (z $\sim$ 0.03-0.04) samples to study statistical properties of low-mass galaxies, their space distribution and luminosity function, their star formation rate and to find more extremely low-metallicity galaxies.

There are at present several large BCG samples on the Northern sky at high latitude. One of them includes $\sim$ 240 objects in the region of the SBS survey (Markarian et al. [1983], [1987]) $\alpha$ = 7^h 40^m - 17^h 20^m, $\delta$ = $49^{\circ} - 61.2^{\circ}$within an area of about 1000 square degrees (Izotov et al. [1993a], [1993b], Pustilnik et al. [1995]) and is based on ELGs selected by the SBS and the First Byurakan survey (FBS; Markarian catalog; Markarian et al. [1989]; Mazzarella & Balzano [1986]) (hereafter this sample is referred to as the SBS sample). Another sample is based on the galaxies found in the Case Northern sky objective prism survey (see Pesch et al. [1995] for the most recent reference). This BCG sample in the sky region of $\alpha = 8^h - 16^h$, $\delta = +29^{\circ} - +38^{\circ}$ was recently presented by Ugryumov ([1997]). The sample was created on results of follow-up spectroscopy with the 6m Russian telescope by Ugryumov et al. ([1998]) and incorporates the results of earlier spectroscopy of Case ELG candidates by Augarde et al. ([1987]), Weistrop & Downes ([1988], [1991]) and Salzer et al. ([1995]) (hereafter this sample is referred to as the Case sample). A third sample of BCGs of Popescu et al. ([1997], [1998]) contains about 200 BCGs distributed in four regions of the northern sky and is based on a sample selected on the Hamburg Quasar Survey (HQS; Hagen et al. [1995]) digitised IIIa-J plates.

Between the above mentioned surveys still exists a large area in the sky not yet totally covered (see Fig. 6). A minor part of this region was covered by the survey of Popescu et al. (1997, 1998). It was tempting to cover this gap with a new ELG survey using also the HQS objective prism plates, thus extending the survey of Popescu et al. ([1997], [1998]). The new project -- the Hamburg/SAO survey for emission-line galaxies (HSS) -- aims to create a new large and deep BCG sample in the area $7^h20^m < \alpha < 17^h40^m$, $35^\circ < \delta < 50^\circ$.

In this paper we describe the general outline of the project and present first results, which are used to improve the selection efficiency for the following parts of the survey. In Section 2 the selection criteria are discussed, in Section 3 the follow-up spectral observations and data reduction are described. We present the results and analysis of spectrophotometric observations in Section 4. The efficiency of the survey in BCG detection, and future perspectives are discussed in Section 5. In Section 6 we summarise the first results of the survey. Throughout this paper a Hubble constant of H₀ = 75 km$\,$s^-1 Mpc^-1 is used.