We have obtained V and I-band photometry for 1886 stars down to I = 27 and V = 28 in the field of the dwarf irregular galaxy WLM , using deep STIS CL and LP -band images taken in the parallel mode with the Hubble Space Telescope .
The photometry is used to study the horizontal branch identified in WLM .
The horizontal branch , extending blueward from the red giant clump , is an unambiguous signature of an old population .
We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of \simeq 1 Mpc using STIS , with relatively short exposure times .
From the VI color–magnitude diagram we obtain an accurate distance modulus ( m - M ) _ { 0 } = 24.95 \pm 0.13 for WLM by using the V-magnitude of the horizontal branch , and by adopting E ( V - I ) = 0.03 .
The implications are : 1 ) WLM formed stars at high redshift , and 2 ) the old population of WLM can be representative of a protogalactic fragment , related to those accreted to form the Milky Way halo .