  In view of both the size of its gap and the previously reported asymmetries
and near-infrared spiral arms, the transition disk of the Herbig Fe star HD
142527 constitutes a remarkable case study. This paper focuses on the
morphology of the outer disk through ALMA observations of $^{12}$CO J=2-1,
$^{12}$CO J=3-2 and $^{13}$CO J=2-1. Both $^{12}$CO J=2-1 and $^{12}$CO J=3-2
show spiral features of different sizes. The innermost spiral arm (S1) is a
radio counterpart of the first near-infrared spiral observed by Fukagawa et al.
(2006), but it is shifted radially outward. However, the most conspicuous CO
spiral arm (S2) lies at the outskirts of the disk and had not been detected
before. It corresponds to a cold density structure, with both brightness and
excitation temperatures of order 13$\pm$2 K and conspicuous in the $^{12}$CO
J=2-1 peak-intensity map, but faint in $^{12}$CO J=3-2. There is also a faint
counterarm (S3), point-symmetrical of S2 with respect to the star. These three
spirals are modelled separately with two different formulae that approximate
the loci of density maxima in acoustic waves due to embedded planets. S1 could
be fit relatively well with these formulae, compared to S2 and S3. Alternative
scenarios such as gravitational instability or external tidal interaction are
discussed. The impact of channelization on spectrally and spatially resolved
peak intensity maps is also briefly addressed.
