We present the results of near-infrared H band (1.65 mu m) imaging of
11 BL Lac objects with redshifts ranging from z = 0.05 to 0.9. We are
able to clearly detect the host galaxy in seven low redshift (z less t
han or equal to 0.24) BL Lacs, while the four unresolved BL Lacs have
either high or unknown redshift. The galaxies hosting the low redshift
BL Lacs are large (average bulge scale length R(e) = 8.8+/-9.9 kpc) a
nd luminous (average M(H) = -25.8+/-0.5), i.e. slightly brighter than
the typical galaxy luminosity L (M*(H) = -25.0+/-0.2), and of similar
luminosity to or slightly fainter than brightest cluster galaxies (M(
H) = -26.3+/-0.3). The average optical/near-infrared colour and colour
gradient of the BL Lac hosts (R-H = 2.2+/-0.5; Delta(R-H)/Delta(log r
) = -0.09+/-0.04) are consistent with the hosts being normal elliptica
ls, indicating that the nuclear activity has only a marginal effect on
the star formation history and other properties of the hosts. The BL
Lac hosts appear slightly less luminous than those of higher redshift
flat spectrum radio quasars. The nucleus-to-galaxy luminosity ratio of
the BL Lacs is similar to that of low redshift radio galaxies and con
sistent with what found in previous optical studies of BL Lacs. Howeve
r, it is smaller that that found for flat spectrum radio quasars, sugg
esting there is a difference in the intrinsic brightness of the nuclea
r source or in the Doppler beaming factor between the two types of bla
zars.