int sisend = 0;
void setup() {
        Serial.begin(9600); 
        pinMode(13, OUTPUT);
}

void loop() {
        if (Serial.available() > 0) {
                sisend = Serial.read();
                if(sisend==97){
                  digitalWrite(13, HIGH);
                }
                if(sisend==98){
                  digitalWrite(13, LOW);
                }
        }
}

/*
>>> import serial
>>> ser=serial.Serial("COM4")
>>> ser.write(b'a')
1
>>> ser.write(b'b')
1
>>> ser.close()
*/