  We address the issue of the production of Lyman continuum photons by T Tauri
stars, in an attempt to provide constraints on theoretical models of disc
photoionisation. By treating the accretion shock as a hotspot on the stellar
surface we show that Lyman continuum photons are produced at a rate
approximately three orders of magnitude lower than that produced by a
corresponding black body, and that a strong Lyman continuum is only emitted for
high mass accretion rates. When our models are extended to include a column of
material accreting on to the hotspot we find that the accretion column is
extremely optically thick to Lyman continuum photons. Further, we find that
radiative recombination of hydrogen atoms within the column is not an efficient
means of producing photons with energies greater than 13.6eV, and find that an
accretion column of any conceivable height suppresses the emission of Lyman
continuum photons to a level below or comparable to that expected from the
stellar photosphere. The photospheric Lyman continuum is itself much too weak
to affect disc evolution significantly, and we find that the Lyman continuum
emitted by an accretion shock is similarly unable to influence disc evolution
significantly. This result has important consequences for models which use
photoionisation as a mechanism to drive the dispersal of circumstellar discs,
essentially proving that an additional source of Lyman continuum photons must
exist if disc photoionisation is to be significant.
