  We establish an effective theory for heavy fermion compounds close to a zero
temperature Anti-Ferromagnetic (AF) transition. Coming from the heavy Fermi
liquid phase across to the AF phase, the heavy electron fractionalizes into a
light electron, a bosonic spinon and a {\it new} excitation: a spinless
fermionic field. Assuming this field acquires dynamics and dispersion when one
integrates out the high energy degrees of freedom, we give a scenario for the
volume of its Fermi surface through the phase diagram. We apply our theory to
the special case of YbRh$_2$(Si$_{1-x}$ Ge$_x$)$_2$ where we recover, within
experimental resolution, several low temperature exponents for transport and
thermodynamics.
