1 Introduction

The M81 group of galaxies, a condensation in the Coma-Sculptor cloud (Tully 1987), is one of the nearest groups to our own. The prominent members include the large spiral M81, the two peculiar galaxies M82 and NGC3077, and a few small spirals. The combination of richness, proximity, and high galactic latitude ( $b \approx 40\hbox{$^\circ$ }$) makes this group an ideal probe for its dwarf galaxy population. Moreover, the M81 group plays an important role in bridging the gap between the investigation of dwarfs in the Local Group and in nearby clusters. Dwarf galaxies are prime targets in the context of such important astronomical issues as cosmic structure formation, primordial chemical abundances, dark matter, galaxy evolution, and star formation.

In an extensive photographic survey, Börngen, Karachentseva, Karachentsev and co-workers have searched for dwarf galaxies in the M81 group (Börngen & Karachentseva 1982; Börngen et al. 1982; Börngen et al. 1984; Karachentseva et al. 1984; Karachentseva et al. 1985). The paper by Karachentseva et al. (1985) lists 40 certain, possible or probable members (where membership was found questionable for 14 of them); large scale photographs from the 6m telescope are given there for 38 objects. Bremnes et al. (1998; hereafter BBP98) provide CCD photometric data and structure parameters for several dwarfs from this list. Searches for HI emission from optically detected M81 dwarf galaxy candidates have been performed by vanDriel et al. (1998) and Huchtmeier & Skillman (1998). Miller & Hodge (1994) investigated HII regions of five members of the M81 group to derive their present star formation rates.

The photographic search for dwarfs in the M81 group was essentially based on simply surveying Tautenburg Schmidt plates by eye. Despite the success of this method, one may doubt the completeness at low surface brightness. For a more complete survey, the use of large-scale CCDs, more sensitive photographic emulsions, or the method of co-addition of Schmidt plates seems more promising.

A CCD survey over 40 square degrees of the M81 region has been performed by Caldwell et al. (1998) with the Burrell Schmidt telescope to search for analogs of Local Group dE's. Caldwell et al. note that their survey goes probably deeper than the photographic search, but they do not provide accurate survey limits. They recovered all previously known dwarfs in their survey field, along with several new candidates. However, data are given only for one of their candidates (F8D1 in their notation), which is discussed there in detail. Karachentseva & Karachentsev (1998) made an attempt to increase the completeness of the sample of nearby galaxies by the use of the POSS-II and ESO/SERC plates. Their list of new dwarf galaxy candidates includes 6 further probable members of the M81 group.

The co-addition (stacking) of several Schmidt plates of the same field provides an interesting alternative for a new, more sensitive photographic survey. Stacking of Schmidt plates is known to be a powerful method for the search of LSB objects in wide fields (e.g., Schweitzer & Seitzer 1988; Hawkins 1991; Malin & Carter 1993; Kemp & Meaburn 1995; Knox et al. 1998; Brunzendorf & Meusinger 1999a; Meusinger et al. 1999).

The present paper is concerned with the search for LSB dwarf galaxies based on a deep image obtained from the quality-weighted digital stacking of a large number of Tautenburg Schmidt plates. The survey field is centered on M81, the size of the field amounts to 10 square degrees. The M81 group is, of course, much more extended (see e.g., van Driel et al. 1998, their Fig.1); the precise extent is not well determined (de Vaucouleurs 1975).

In the following section, we describe the observational material. The plate stacking is the subject of Sect.3. Section4 is concerned with the search procedure. Images and surface brightness profiles for the new candidates and for previously known LSB dwarfs in the field are presented in Sect.5. Conclusions are given in Sect.6. Throughout this paper, we assume a distance of 3.6Mpc for the M81 group (Madore et al. 1993), implying a linear scale of 1.05kpcarcmin^-1.