Host galaxy and close environment of BL lacertae objects

High-resolution R-band images for 16 BL Lacertae (BL Lac) objects are reported. The data allow us to study the properties of the host galaxies and the close environment with sub-arcsec resolution. We are able to detect the surrounding nebulosity for 11 sources, while four remain unresolved. One case (0235 + 164) is complex and it is considered in detail. We discuss the average properties of host galaxies, including a number of objects previously investigated with the instrumentation and analysis procedure. It is found that in all studied cases, the host galaxies are elliptical or bulge-dominated systems. Although for some objects the presence of a disc galaxy cannot be ruled out, there are no convincing justifications for preferring disc rather than elliptical models. The average absolute magnitude of host galaxies is M(R) = -23.5 (H-0 = 50 km s(-1) kpc(-1) and q(0) = 0). For six objects we have detected companions, which are often resolved and are located at a projected distance from the BL Lac object of about 10-40 kpc. This gives support to the idea that interactions may be important to the BL Lac phenomenon, and is consistent with the finding that many BL Lacs are located in groups of galaxies.