We have searched for corotations ( CR ) in three Southern grand design spiral galaxies : NGC 1365 , NGC 1566 and NGC 2997 .
We have also introduced a method of quantifying errors in the phase diagram used to detect CR .

We established the m=2 pattern CR at 12.1 kpc , 9.4 kpc and 7 kpc , for NGC 1365 , NGC 1566 and NGC 2997 , respectively .
By using published rotation curves , we could determine spiral pattern angular speeds of 25.0 km/sec/kpc , 12.2 km/sec/kpc and 17.6 km/sec/kpc , respectively .

A three armed component has been detected in NGC 2997 , with the CR placed at 8.7 kpc with a pattern angular speed \Omega _ { CR _ { 3 } } = 12.7 km/sec/kpc .

An m=1 component was detected in NGC 1566 .
We warily locate the CR at 7.1 kpc , with a pattern angular speed \Omega _ { CR } \backsimeq 16.6 km/sec/kpc .
This pattern does not present ILR .

Ages have been determined by studying the radial density profile of the m=2 Fourier components in g ( newly formed stars ) and i ( perturbing SDW supported by the disk of old stars ) , aided by the global aspect of the real spiral pattern in comparison with numerical simulations .
The pattern is \thicksim 1200 Myr old in NGC 1365 , \thicksim 800 Myr old in NGC 1566 and younger than 80 Myr in NGC 2997 .