  The classical Rayleigh-B\'{e}nard convection (RBC) system is known to exhibit
either subcritical or supercritical transition to convection in the presence or
absence of rotation and/or magnetic field. However, the simultaneous exhibition
of subcritical and supercritical branches of convection in plane layer RBC
depending on the initial conditions, has not been reported so far. Here, we
report the phenomenon of simultaneous occurrence of subcritical and
supercritical branches of convection in overstable RBC of electrically
conducting low Prandtl number fluids (liquid metals) in the presence of an
external uniform horizontal magnetic field and rotation about the vertical
axis. Extensive three dimensional (3D) direct numerical simulations (DNS) and
low dimensional modeling of the system, performed in the ranges $750 \leq
\mathrm{Ta} \leq 3000$ and $0 < \mathrm{Q} \leq 1000$ of the Taylor number
($\mathrm{Ta}$, strength of the Coriolis force) and the Chandrasekhar number
($\mathrm{Q}$, strength of the Lorenz force) respectively, establish the
phenomenon convincingly. Detailed bifurcation analysis of a simple three
dimensional model derived from the DNS data reveals that a supercritical Hopf
bifurcation and a subcritical pitchfork bifurcation of the conduction state are
responsible for this. The effect of Prandtl number on these transitions is also
explored in detail.
