The narrow-band ∆a photometric system measures the flux depression at λ5200 Å by comparing the flux at the band centre to adjacent regions. It has been shown that virtually all peculiar stars with magnetic fields (mCPs) have significant positive ∆a values (of up to +100 mmag), whereas Be/Ae/B[e] and metal weak stars (including lambda Bootis types) exhibit significant negative values of ∆a. By using this photometric system we are therefore able to detect chemically peculiar (CP), emission types and metal-weak stars in an efficient way. The poster presented the first results of our survey of the Small Magellanic Cloud. In the selected field we found only 0.5 % of bona fide CP stars against 15 % in our Galaxy.