“…The principal transverse bivector bases {W i , U i , V i } can be obtained as (11) where {W i } is given in theorem 2. Once the transverse bivector bases {W T , U T , V T } are known, we can look for the transverse frames {l T , n T , m T ,m T } associated with them by (6). To obtain them, we could apply to the bivectors {W T , U T , V T } the covariant method to determine the principal directions of a 2-form [14] (see also [10] [11]), but here we opt by an alternative procedure based on proposition 2: starting from an arbitrary null tetrad {l, k, m,m} we will obtain the Weyl orthonormal frame {e α } and, from it, we derive the null transverse frames {l T , k T , m T ,m T }.…”