Subnatural-width resonances due to coherent effects in optical pumping at h
yperfine (hf) transitions of Rb are investigated using a linearly or circul
arly polarized single-frequency diode laser beam. The sign and amplitude of
the resonances in the fluorescence are measured by scanning the laser freq
uency over the hf transitions of the Rb D-1 and D-2 lines. The fluorescence
perpendicular to the laser beam is detected as a function of magnetic fiel
d parallel to the beam propagation and scanned around zero value. In the ca
se of linear polarization of the field, dark resonances are observed for hf
transitions with F-g --> F-e = F-g - 1 and F-g --> F-e = F-g, and bright r
esonances are observed for transitions with F-g --> F-e = F-g + 1. This is
in agreement with the recently developed theories. For all (except F-g = 2
--> F-e = 3, Rb-85, D-1 line) hf transitions the sign of the resonances rev
erses when the polarization of the field changes to circular. The amplitude
s of the resonances correspond to the probability of the hf transitions and
to the calculated loss rate of the population to the ground hf level not e
xcited by the laser field. The observed narrow resonances (FWHM: few tens o
f kHz) and their sensitivity to weak magnetic fields are interesting for hi
gh-resolution spectroscopy and for development of simple and precise magnet
ometers.