I mentioned him in relation to 5000 Dollars on an Ace (see note 4, there's a link to the song):
Ronny had several great hits in my country, Holland. In fact he was a bigger star in Holland in the 60s than his rival (and in Germany far more successful) Freddy Quinn.
People often spoke about Ronny & Freddy as if they were some kind of twin artists (in the same misleading way they spoke about Elvis & Cliff)
His biggest hit in Holland was Kleine Annabel, which was several weeks at number one. His star declined with the breakthrough of 'pop music', all of a sudden all hits were sung in English. His most enduring success in Germany, Sierra Madre del sur, never hit the charts in Holland.
He had a great voice and also occasionally sang in English.
Sierra Madre del Sur: