Ghost systems: a vertex algebra point of view

Abstract Fermionic and bosonic ghost systems are defined each in terms of a single vertex algebra which admits a one-parameter family of conformal structures. The observation that these structures are related to each other provides a simple way to obtain character formulae for a general twisted module of a ghost system. The U (1) symmetry and its subgroups that underlie the twisted modules also define an infinite set of invariant vertex subalgebras. Their structure is studied in detail from a W algebra point of view with particular emphasis on Z N -invariant subalgebras of the fermionic ghost system.