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We have obtained deep photometry in two 1x1 degree fields covering the closepair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the areain between. We find that both systems are significantly larger than indicatedby previous measurements based on shallower data and also significantlyelongated. With half-light radii of r_h=4'.6 +- 0'.8 (206 +- 36 pc) andr_h=2'.6 +- 0'.6 (133 +- 31 pc), respectively, they are now well within thephysical size bracket of typical Milky Way dSph satellites. Their ellipticitiesof epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The largespatial extent of our survey allows us to search for extra-tidal features withunprecedented sensitivity. The spatial distribution of candidate red giantbranch and horizontal branch stars is found to be non-uniform at the ~3 sigmalevel. This substructure is aligned along the direction connecting the twosystems, indicative of a possible `bridge' of extra-tidal material. Fitting thestellar distribution with a linear Gaussian model yields a significance of 4sigma for this overdensity, a most likely FWHM of ~16 arcmin and a centralsurface brightness of ~32 mag arcsec^{-2}. We investigate different scenariosto explain the close proximity of Leo IV and Leo V and the possible tidalbridge between them. Orbit calculations demonstrate that they are unlikely tobe remnants of a single disrupted progenitor, while a comparison withcosmological simulations shows that a chance collision between unrelatedsubhalos is negligibly small. Leo IV and Leo V could, however, be a bound`tumbling pair' if their combined mass exceeds 8 +- 4 x 10^9 M_sun. Thescenario of an internally interacting pair appears to be the most viableexplanation for this close celestial companionship. (abridged)

The enigmatic pair of dwarf galaxies Leo IV and Leo V: coincidence or common origin?