At some point in the mid-late 1990's I borrowed this book from my local library (UK).
Details that I can remember (not much to go on, I know):
- It was from the older childrens or teenage section of the library.
- Children (two - I think) are recruited into a secret organisation.
- Purpose of organisation was to resist a specific alien threat to earth
- The threat was a covert one - the general population was ingorant of any threat and aliens could pass as human (not sure if due to disguise or not).
- At some point I think they visited a space station.
- Membership of the organisation was shown through a brand/tattoo of a "black star" which could appear and disappear at the will of the bearer. Not sure if on arm or forehead. Suspect a black star may have featured on the cover in some way.
Please let me know if you think you know what this book might be. The black stars are what I remember most vividly.