  We compute the conformal anomaly a-coefficient for some non-unitary (higher
derivative or non-gauge-invariant) 6d conformal fields and their
supermultiplets. We use the method based on a connection between 6d
determinants on S^6 and 7d determinants on AdS_7. We find, in particular, that
(1,0) supermultiplet containing 4-derivative gauge-invariant conformal vector
has precisely the value of a-anomaly as attributed in arXiv:1506.03807 (on the
basis of R-symmetry and gravitational 't Hooft matching) to the standard (1,0)
vector multiplet. We also show that higher derivative (2,0) 6d conformal
supergravity coupled to exactly 26 (2,0) tensor multiplets has vanishing
a-anomaly. This is the 6d counterpart of the known fact of cancellation of the
conformal anomaly in the 4d system of N=4 conformal supergravity coupled to 4
vector N=4 multiplets. In the case when 5 of tensor multiplets are chosen to be
ghost-like and the conformal symmetry is spontaneously broken by a quadratic
scalar constraint the resulting IR theory may be identified with (2,0) Poincare
supergravity coupled to 21=26-5 tensor multiplets. The latter theory is known
to be special: it is gravitational anomaly free and results upon
compactification of 10d type IIB supergravity on K3.
