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 S6 and 7d determinants
on AdS7. 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 anomaly 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 aanomaly
(and also vanishing Casimir energy on 5-sphere). 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) Poincar´e
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