  We present VLT spectroscopic observations of Gamma-Ray Burst (GRB) 990705
host galaxy and highlight the benefits provided by the prompt-phase features of
GRBs to derive the redshifts of the latter. In the host spectrum, we indeed
detect an emission feature which we attribute to the [OII] 3726/3729 Angstrom
doublet and derive an unambiguous redshift z=0.8424+/-0.0002 for this galaxy.
This is in full agreement with the value z~0.86+/-0.17 previously derived using
a transient absorption edge discovered in the X-ray spectrum of GRB990705. This
burst is therefore the first GRB for which a reliable redshift was derived from
the prompt phase emission itself, as opposed to redshift determinations
performed using putative host galaxy emission lines or intestellar absorption
lines in the GRB afterglows. Deep and high resolution images of the host of
GRB990705 with the HST/STIS camera reveal that the burst occured in a nearly
face-on Sc spiral galaxy typical of disk-dominated systems at 0.75<z<1.
Assuming a cosmology with Ho=65km/s/Mpc, Omega_m=0.3 and Omega_lambda=0.7, we
derive an absolute B magnitude M_B=-21.75 for this galaxy and a star formation
rate SFR ~ 5-8~Msol/yr. Finally, we discuss the implications of using X-ray
transient features to derive GRB redshifts with larger burst samples, and
especially examine the case of short and dark-long GRBs.
