  We discuss the dependence of the phase diagram of a hypothetical isotope of
helium with nuclear mass less than 4 atomic mass units. We argue that with
decreasing nucleus mass, the temperature of the superfluid phase transition
(about 2.2 K in real He-4) increases, while that of the liquid-gas critical
point (about 5.2 K in real He-4) decreases. We discuss various scenarios that
may occur when the two temperatures approach each other and the order
parameters of the superfluid and the liquid-gas phase transitions interact with
each other. The simplest scenario, in which both order parameters become
critical at particular values of the nuclear mass, temperature, and pressure,
can be ruled out through on an analysis of the Landau theory. We argue that in
the most likely scenario, as the nuclear mass decreases, first, a tricritical
point appears on the line separating the superfluid and the normal fluid phase,
then the critical point disappears under the first-order part of superfluid
phase transition line, and in the end the tricritical point disappears. The
last change in the phase diagram occurs when the two-body scattering length
crosses zero, which corresponds to the nuclear mass of about 1.55 u. We develop
a quantitative theory that allows one to determine the phase diagram in the
vicinity of this point. Finally, we discuss several ways to physically realize
such liquids.
