I realize it doesn't sound like that much of a stretch to call an airline stock toxic. But discount European carrier Ryanair Holdings ( RYAAY) is just now starting to show some technical weakness. Investors could get their first distinct sell (or short) signal in Ryanair this week. Like BHP Billiton, Ryanair has been a strong performer for the last couple quarters - shares of the $11 billion carrier have climbed by more than 32% in those last six months. But Ryanair is showing weakness thanks to a rounding top in shares. Like the name implies, a rounding top looks like an upside down parabola in a stock's price action. It represents a gradual shift of power from buyers to sellers. In Ryanair's case, the pattern triggers on a move down through $38. Momentum is important on this stock as well: RSI had show RYAAY in overbought territory as the calendar flipped over to 2013. Contrary to popular belief, an overbought reading in momentum isn't a bearish signal in and of itself (overbought stocks statistically tend to go even more overbought in the short-term), but the fact that momentum is rolling over simultaneously is bearish. I'd be looking to take gains on a slip below $38.