Kaonium is an exotic atom consisting of a bound state of a positively charged and a negatively charged kaon. Kaonium has not been observed experimentally and is expected to have a short lifetime on the order of 10−18 seconds. ... Found on http://en.wikipedia.org/wiki/Kaonium
1) Exotic atom 
Kaonium is an exotic atom consisting of a bound state of a positively charged and a negatively charged kaon. Kaonium has not been observed experimentally and is expected to have a short lifetime on the order of 10−18 seconds. ...