Always use an ART-approved lock to secure your two-wheeler. Always secure it to a fixed point as well. If you have a bike with a ring lock, always use an extra ART-approved lock to secure the bike.
If you use a chain with a loose ring lock, make sure that both have the same number of stars. After all, even here the lock is as good as the weakest link.
Sometimes you can buy an extra connection option with the ring lock of your bike, a so-called “plug-in”. For sufficient protection against theft, you should make sure that this connection option also has ART approval.