following information was sent to me by William Cunningham:
To type umlaute and ß with a compose key in Kubuntu Linux:
1. Open the K menu and select "System Settings"
2. Under "Personal" select "Regional and Language"
3. Select "Keyboard Layout" from the menu on the left
4. Check the box marked "Enable Keyboard Layouts"
5. Select the "Advanced" tab from the top
6. Scroll down to the option marked "Compose Key Position"
7. Pick any of the optional modifiers as a compose key
Then the key sequences with this compose key are listed in a handy reference website:
Linux Compose Key Sequences