  We propose to infer ionising continuum leaking properties of galaxies by
looking at their Lyman-alpha line profiles. We carry out Lyman-alpha radiation
transfer calculations in two models of HII regions which are porous to ionising
continuum escape: 1) the so-called "density bounded" media, in which massive
stars produce enough ionising photons to keep the surrounding interstellar
medium transparent to the ionising continuum, i.e almost totally ionised, and
2) "riddled ionisation-bounded" media, surrounded by neutral interstellar
medium, but with holes, i.e. with a covering factor lower than unity. The
Lyman-alpha spectra emergent from these configurations have distinctive
features: 1) a "classical" asymmetric redshifted profile in the first case, but
with a small shift of the maximum of the profile compare to the systemic
redshift (Vpeak < 150 km/s); 2) a main peak at the systemic redshift in the
second case (Vpeak = 0 km/s), with, as a consequence, a non-zero Lyman-alpha
flux bluewards the systemic redshift. Assuming that in a galaxy leaking
ionising photons, the Lyman-alpha component emerging from the leaking star
cluster(s) dominates the total Lyman-alpha spectrum, the Lyman-alpha shape may
be used as a pre-selection tool to detect Lyman continuum (LyC) leaking
galaxies, in objects with well determined systemic redshift, and high spectral
resolution Lyman-alpha spectra (R >= 4000). The examination of a sample of 10
local starbursts with high resolution HST-COS Lyman-alpha spectra and known in
the literature as LyC leakers or leaking candidates, corroborates our
predictions. Observations of Lyman-alpha profiles at high resolution should
show definite signatures revealing the escape of Lyman continuum photons from
star-forming galaxies.
