  We show the existence and investigate the location of the special point (SP)
in which hybrid neutron star mass-radius (M-R) curves have to cross each other
when they belong to a class of hybrid equation of state (EoS) constructed with
generic constant--speed--of--sound (CSS) quark matter models for which the
onset deconfinement is varied. We demonstrate that for a three-parameter CSS
model the position of the SP in the M-R diagram is largely independent of the
choice of the hadronic EoS, but in dependence on the stiffness of the quark
matter EoS it spans a region that we identify. We find that the difference
between the maximum mass and the SP mass depends on the mass at the onset of
deconfinement so that an upper limit of $0.19~M_\odot$ for this difference is
obtained from which a lower limit on the radius of hybrid stars is deduced.
Together with a lower limit on the radius of hadronic stars, derived from a
class of reasonably soft hadronic EoS including hyperons, we identify a region
in the M-R diagram which can be occupied only by hybrid stars. Accordingly, we
suggest that a NICER radius measurement on the massive pulsar PSR J0740+6620 in
the range of 8.6-11.9 km would indicate that this pulsar is a hybrid neutron
star with deconfined quark matter in the inner core.
