Circumstellar discs with large dust depleted cavities and vigorous accretion
onto the central star are often considered signposts for (multiple) giant
planet formation. In this letter we show that X-ray photoevaporation operating
in discs with modest (factors 3-10) gas-phase depletion of Carbon and Oxygen at
large radii (> 15 AU) yield the inner radius and accretion rates for most of
the observed discs, without the need to invoke giant planet formation. We
present one-dimensional viscous evolution models of discs affected by X-ray
photoevaporation assuming moderate gas-phase depletion of Carbon and Oxygen,
well within the range reported by recent observations. Our models use a
simplified prescription for scaling the X-ray photoevaporation rates and
profiles at different metallicity, and our quantitative result depends on this
scaling. While more rigorous hydrodynamical modelling of mass loss profiles at
low metallicities is required to constrain the observational parameter space
that can be explained by our models, the general conclusion that metal
sequestering at large radii may be responsible for the observed diversity of
transition discs is shown to be robust. Gap opening by giant planet formation
may still be responsible for a number of observed transition discs with large
cavities and very high accretion rate.